Move stuff around a bit:

- Create FileSystem class:
  * Move ServerConfig::CleanFilename to FileSystem::GetFileName and rewrite.
  * Move ServerConfig::ExpandPath to FileSystem.
  * Move ServerConfig::FileExists to FileSystem.
  * Move ServerConfig::StartsWithWindowsDriveLetter to FileSystem.
- Move FileReader to fileutils.cpp and fix documentation.
- Move UserManager::DoBackgroundUserStuff to usermanager.cpp.
Remove the CHARSET entry from ISUPPORT.

- CHARSET was removed in draft-brocklesby-irc-isupport-03 and we
  always used the default value anyway. This has also been removed
  in the latest version of Charybdis.