Merge bitcoin/bitcoin#24831: tidy: add include-what-you-use

9b0a13a2891641a3d12e525cee8ddddb1aa1bc73 tidy: Add include-what-you-use (fanquake)
74cd038e300bfbe2473295fc3b0c3a4f3e853a07 refactor: fix includes in src/init (fanquake)
c79ad935f0412bac3e19a6b925efdb390eb00bd9 refactor: fix includes in src/compat (fanquake)

Pull request description:

  We recently added a [`clang-tidy` job](https://github.com/bitcoin/bitcoin/blob/master/ci/test/00_setup_env_native_tidy.sh) to the CI, which generates a compilation database. We can leverage that now existing database to begin running [include-what-you-use](https://include-what-you-use.org/) over the codebase.

  This PR demonstrates using a mapping_file to indicate fixups / includes that may differ from IWYU suggestions. In this case, I've added some fixups for glibc includes that I've [upstreamed changes for](https://github.com/include-what-you-use/include-what-you-use/pull/1026):
  ```bash
  # Fixups / upstreamed changes
  [
    { include: [ "<bits/termios-c_lflag.h>", private, "<termios.h>", public ] },
    { include: [ "<bits/termios-struct.h>", private, "<termios.h>", public ] },
    { include: [ "<bits/termios-tcflow.h>", private, "<termios.h>", public ] },
  ]
  ```

  The include "fixing" commits of this PR:
  * Adds missing includes.
  * Swaps C headers for their C++ counterparts.
  * Removes the pointless / unmaintainable `//for abc, xyz` comments. When using IWYU, if anyone wants to see / generate those comments, to see why something is included, it is trivial to do so (IWYU outputs them by default). i.e:
  ```cpp
  // The full include-list for compat/stdin.cpp:
  #include <compat/stdin.h>
  #include <poll.h>                  // for poll, pollfd, POLLIN
  #include <termios.h>               // for tcgetattr, tcsetattr
  #include <unistd.h>                // for isatty, STDIN_FILENO
  ```

  TODO:
  - [ ] Qt mapping_file. There is one in the IWYU repo, but it's for Qt 5.11. Needs testing.
  - [ ] Boost mapping_file. There is one in the IWYU repo, but it's for Boost 1.75. Needs testing.

  I'm not suggesting we turn this on the for entire codebase, or immediately go-nuts refactoring all includes. However I think our dependency includes are now slim enough, and our CI infrastructure in place such that we can start doing this in some capacity, and just automate away include fixups / refactorings etc.

ACKs for top commit:
  MarcoFalke:
    review ACK 9b0a13a2891641a3d12e525cee8ddddb1aa1bc73
  jonatack:
    ACK 9b0a13a2891641a3d12e525cee8ddddb1aa1bc73 reviewed changes and run CI output in https://cirrus-ci.com/task/4750910332076032

Tree-SHA512: 00beab5a5f2a6fc179abf08321a15391ecccaa91ab56f3c50c511e7b29a0d7c95d8bb43eac2c31489711086f6f77319d43d803cf8ea458e7cd234a780d9ae69e

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