Support individual channel keys for pre-defined channels.

Support individual channel keys for pre-defined channels.

This patch introduces the new configuration variable "KeyFile" for
[Channel] sections in ngircd.conf. Here a file can be configured for each
pre-defined channel which contains individual channel keys for different
users. This file is line-based and must have the following syntax:

  <user>:<nick>:<key>

<user> and <nick> can contain the wildcard character "*".

Please not that these channel keys are only in effect, when the channel
has a regular key set using channel mode "k"!