Allow trailing comments in config file:

Treat all `#` characters in config files as comments (and remove)
*unless* the `#` is immediately preceded by `\`. Write a warning
to log file when trailing comments are found/ignored in the config
to let operators know that the treatment of trailing `#` has changed.

Fixes #3121

src/ripple/core/impl/Config.cpp

@@ -149,7 +149,7 @@ getIniFileSection (IniFileSections& secSource, std::string const& strSection)
     IniFileSections::mapped_type* smtResult;
     it  = secSource.find (strSection);
     if (it == secSource.end ())
-        smtResult   = 0;
+        smtResult   = nullptr;
     else
         smtResult   = & (it->second);
     return smtResult;