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 some pointless code:

- 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.

- Remove irc::sockets::satouser.

  This helper method was longer than the code it replaced.
Purge docs/rfc from the repository.

These are of no use to 99% of users and anyone who actually wants
to read them should be capable of using Google to find them.
Make various self contained methods static.

- InspIRCd::IsValidMask
- InspIRCd::TimeString