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.

./inspircd --nolog has been repurposed to turn off logging entirely
when used.
Accept a LogLevel instead of an int in logging methods.
Rename LogManager::Log to LogManager::Write to be less repetitive.