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.

src/init.cpp

@@ -925,7 +925,8 @@ bool AppInitParameterInteraction()
         // Special-case: if -debug=0/-nodebug is set, turn off debugging messages
         const std::vector<std::string> categories = gArgs.GetArgs("-debug");
 
-        if (find(categories.begin(), categories.end(), std::string("0")) == categories.end()) {
+        if (std::none_of(categories.begin(), categories.end(),
+            [](std::string cat){return cat == "0" || cat == "none";})) {
             for (const auto& cat : categories) {
                 uint32_t flag = 0;
                 if (!GetLogCategory(&flag, &cat)) {