New configuration option "NoPAM" to disable PAM

New configuration option "NoPAM" to disable PAM

When the "NoPAM" configuration option is set and ngIRCd is compiled
with support for PAM, ngIRCd will not call any PAM functions: all
connection attemps without password will succeed instead and all
connection attemps with password will fail.

If ngIRCd is compiled without PAM support, this option is a dummy
option and nothing changes: the global server password will still be
in effect.