Merge #20685: Add I2P support using I2P SAM

a701fcf01f3ea9a12e869bfa52321302cf68351c net: Do not skip the I2P network from GetNetworkNames() (Vasil Dimov)
0181e244394bd9e68e9f0d44704e7b0fd12a6b1f net: recognize I2P from ParseNetwork() so that -onlynet=i2p works (Vasil Dimov)
b905363fa8b0bb03fe34b53b5410880f42e0af39 net: accept incoming I2P connections from CConnman (Vasil Dimov)
0635233a1e7e8c303073430092afd3e0fb0d927b net: make outgoing I2P connections from CConnman (Vasil Dimov)
9559bd1404fbf74b0d09fe9019a9305cb4e151ce net: add I2P to the reachability map (Vasil Dimov)
76c35c60f338937071bcfad4211ef7254d3830ec init: introduce I2P connectivity options (Vasil Dimov)
c22daa2ecff1acd25426cd46f98f2587d1d324c3 net: implement the necessary parts of the I2P SAM protocol (Vasil Dimov)
5bac7e45e1d3a07115b5ff002d988438fcc92a53 net: extend Sock with a method to check whether connected (Vasil Dimov)
42c779f503eb8437b6232773a4a2472306cc9f3d net: extend Sock with methods for robust send & read until terminator (Vasil Dimov)
ea1845315a109eb105113cb5fbb6f869e1cf010c net: extend Sock::Wait() to report a timeout (Vasil Dimov)
78fdfbea666201b25919dd67454eb04d6a34326f net: dedup MSG_NOSIGNAL and MSG_DONTWAIT definitions (Vasil Dimov)
34bcfab562bac9887ca9c3831cf4fd0ee7f98149 net: move the constant maxWait out of InterruptibleRecv() (Vasil Dimov)
cff65c4a270887ec171293409ab84f5d0d0be7fc net: extend CNetAddr::SetSpecial() to support I2P (Vasil Dimov)
f6c267db3be2d7077fd2bdbd34860eba838dea99 net: avoid unnecessary GetBindAddress() call (Vasil Dimov)
7c224fdac4699a2c4953b33ab423f9cddbf95cf7 net: isolate the protocol-agnostic part of CConnman::AcceptConnection() (Vasil Dimov)
1f75a653dd3b24ba2e4383bf951a6e5a3d5ccbcf net: get the bind address earlier in CConnman::AcceptConnection() (Vasil Dimov)
25605895afe84b1765dd9da9240af22f99489df7 net: check for invalid socket earlier in CConnman::AcceptConnection() (Vasil Dimov)
545bc5f81d60fa6ff7c5cc43a2e9eef82f911466 util: fix WriteBinaryFile() claiming success even if error occurred (Vasil Dimov)
8b6e4b3b23027da263d257b342f5d9a53e4032d5 util: fix ReadBinaryFile() returning partial contents (Vasil Dimov)
4cba2fdafa483cbdb70f581174138ec253c80d48 util: extract {Read,Write}BinaryFile() to its own files (Vasil Dimov)

Pull request description:

  Add I2P support by using the [I2P SAM](https://geti2p.net/en/docs/api/samv3) protocol. Unlike Tor, for incoming connections we get the I2P address of the peer (and they also receive ours when we are the connection initiator).

  Two new options are added:

  ```
    -i2psam=<ip:port>
         I2P SAM proxy to reach I2P peers and accept I2P connections (default:
         none)

    -i2pacceptincoming
         If set and -i2psam is also set then incoming I2P connections are
         accepted via the SAM proxy. If this is not set but -i2psam is set
         then only outgoing connections will be made to the I2P network.
         Ignored if -i2psam is not set. Notice that listening for incoming
         I2P connections is done through the SAM proxy, not by binding to
         a local address and port (default: true)
  ```

  # Overview of the changes

  ## Make `ReadBinary()` and `WriteBinary()` reusable

  We would need to dump the I2P private key to a file and read it back later. Move those two functions out of `torcontrol.cpp`.

  ```
  util: extract {Read,Write}BinaryFile() to its own files
  util: fix ReadBinaryFile() returning partial contents
  util: fix WriteBinaryFile() claiming success even if error occurred
  ```

  ## Split `CConnman::AcceptConnection()`

  Most of `CConnman::AcceptConnection()` is agnostic of how the socket was accepted. The other part of it deals with the details of the `accept(2)` system call. Split those so that the protocol-agnostic part can be reused if we accept a socket by other means.

  ```
  net: check for invalid socket earlier in CConnman::AcceptConnection()
  net: get the bind address earlier in CConnman::AcceptConnection()
  net: isolate the protocol-agnostic part of CConnman::AcceptConnection()
  net: avoid unnecessary GetBindAddress() call
  ```

  ## Implement the I2P [SAM](https://geti2p.net/en/docs/api/samv3) protocol (not all of it)

  Just the parts that would enable us to make outgoing and accept incoming I2P connections.

  ```
  net: extend CNetAddr::SetSpecial() to support I2P
  net: move the constant maxWait out of InterruptibleRecv()
  net: dedup MSG_NOSIGNAL and MSG_DONTWAIT definitions
  net: extend Sock::Wait() to report a timeout
  net: extend Sock with methods for robust send & read until terminator
  net: extend Sock with a method to check whether connected
  net: implement the necessary parts of the I2P SAM protocol
  ```

  ## Use I2P SAM to connect to and accept connections from I2P peers

  Profit from all of the preceding commits.

  ```
  init: introduce I2P connectivity options
  net: add I2P to the reachability map
  net: make outgoing I2P connections from CConnman
  net: accept incoming I2P connections from CConnman
  net: recognize I2P from ParseNetwork() so that -onlynet=i2p works
  net: Do not skip the I2P network from GetNetworkNames()
  ```

ACKs for top commit:
  laanwj:
    re-ACK a701fcf01f3ea9a12e869bfa52321302cf68351c
  jonatack:
    re-ACK a701fcf01f3ea9a12e869bfa52321302cf68351c reviewed diff per `git range-diff ad89812 2a7bb34 a701fcf`, debug built and launched bitcoind with i2pd v2.35 running a dual I2P+Torv3 service with the I2P config settings listed below (did not test `onlynet=i2p`); operation appears nominal (same as it has been these past weeks), and tested the bitcoind help outputs grepping for `-i i2p` and the rpc getpeerinfo and getnetworkinfo helps

Tree-SHA512: de42090c9c0bf23b43b5839f5b4fc4b3a2657bde1e45c796b5f3c7bf83cb8ec6ca4278f8a89e45108ece92f9b573cafea3b42a06bc09076b40a196c909b6610e

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