Implement new configuration option "AllowedChannelTypes"

Implement new configuration option "AllowedChannelTypes"

The new configuration variable "AllowedChannelTypes" lists all allowed
channel types (channel prefixes) for newly created channels on the local
server. By default, all supported channel types are allowed.

If set to the empty string, local clients can't create new channels at
all, which equals the old "PredefChannelsOnly = yes" setting.

This patch deprecates the "PredefChannelsOnly" variable, too, but it is
still supported and translated to the apropriate "AllowedChannelTypes"
setting. When "PredefChannelsOnly" is processed, a warning message is
logged.

Closes bug #152.