Allow ngIRCd to detect services connected to an "virtual services server".

Introduce a new configuration variable "ServiceMask" in SERVER blocks to
define a mask matching nick names that should be treated as services.
Regular servers don't need this parameter (leave it empty, the default),
but you should set it to "*Serv" when connection ircservices, for example.

This patch allows ngIRCd to detect services, it doesn't change the
functionality: you only get different log messages ;-)