fixed MODE_REGISTERED bug; introduced MCP_REGISTERED

* fixed MODE_REGISTERED bug: srvx was sending a blank mode change on channel unregistrations; we now have a srvx.conf setting that dictates whether or not to use MODE_REGISTERED (which has no corresponding letter on P10 networks!) - also, we now set MODE_REGISTERED in newly registered channels (which we werent before)

* introduced MCP_REGISTERED: we now don't let channels that are registered play with the registered mode (usually +r on bahamut networks) via chanserv user-level commands (ie: !mode -r)

git-archimport-id: srvx@srvx.net--2004-srvx/srvx--devo--1.3--patch-68