Enhance "ServiceMask" to handle a list of masks

The "ServiceMask" variable in "Server" blocks now can handle more than
one mask using the new MatchCaseInsensitiveList() function.

This makes marking "service clients" much more specific, which is a
good thing per se, but which is the prerequisite for reasonably
blocking these nick names, too (see commit a6dd2e3 for details).