Remove the size argument from IsChannel and IsNick.

There was only one case (which was probably an error) where these
methods were not set to their ServerLimits value.
Merge pull request #543 from SaberUK/master+sizeparam-removal

Remove the size argument from IsChannel and IsNick.
Deduplicate hex string creation code
irc::stringjoiner cleanup

- Get rid of unused constructors
- signed -> unsigned
- return const ref from GetJoined()
irc::tokenstream and irc::sepstream cleanup

irc::sepstream does not require virtual methods
Remove unused UserManager::ServerPrivmsgAll()
Get rid of the NICKForced extension

Don't run OnUserPreNick when the nick change is forced
Change the signature of User::ForceNickChange() to accept const std::string& instead of const char*
Added macro to allow simpler logic in functions with the need to vsnprintf
Replaced vsnprintf with VAFORMAT pretty much everywhere.