Improve help text and behavior of RPC-logging

A) The changes in behavior are as follows:
1. Introduce logging category "none" as alias of "0" for
   both RPC-logging and bitcoind "-debug" parameter.
2. Same as "0" is given to argument of "-debug",
   if "none" or "0" is given to <include>, all other given logging
   categories are ignored. The same is true for <exclude>.
   (Before this PR, "0" was accepted but just be ignored itself.)

B) The changes in the help text are as follows:
1. Add a descrption about the evaluation order of <include> and
   <exclude> to clarify how debug loggig categories to be set.
2. Delete text that describe restriction about libevent because
   it's already allowed libevent logging to be updated during runtime.
3. Add a description for category "all", "1", "none" and "0".
4. Add "optional" to the help text of <include> and <exclude>.
5. Add missing new lines before "Argument:".
6. This RPC always returns all logging categories with status.
   Fix the help text to match this behavior.

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