Implemented new "secure clients only" channel mode: +z

Implemented new "secure clients only" channel mode: +z

Only clients using a SSL encrypted connection to the server are
allowed to join such a channel.

But please note three things:

a) already joined clients are not checked when setting this mode,
b) IRC operators are always allowed to join every channel, and
c) remote clients using a server not supporting this mode are not
   checked either and therefore always allowed to join.