Move LogLevel enum from filelogger to logger.
Move LogHeader to LogStream.
Purge a ton of code related to non <log> related logging.

- InspIRCd::OpenLog
- ./inspircd --logfile [file]

This is no longer needed now we have (since 1.2) custom logging.
Accept a LogLevel instead of an int in logging methods.
Fix spacing in calls to LogManager::Log.
Convert XLine::Displayable to return a std::string.
Merge pull request #522 from SaberUK/master+xline-displayable

Convert XLine::Displayable to return a std::string.
Merge pull request #545 from SaberUK/master+logging-cleanup

Clean up the logging system (part 1 of 2).