refactored match_ipmask() methods

[patrick-irc]

int match_ipmask(char *mask, char *ip)

should be globally available, so I can use it to check if the SASLService sends an IPv6 in a configured range.

And therefore

int match_ipmask(char *mask, aClient *cptr, int maskwithusername)

should be renamed to match_ipmask_client.