Add "dummy client" support.

src/hash.h (FLAGS_DUMMY): New flag for non-propagated clients.
  (IsDummy): Check it.

src/helpfile.c (send_message): Allow sending to dummy clients.
  (send_message_type): Likewise.

src/proto-bahamut.c (irc_user): Bail if user is NULL or has a dummy nickname.
  (deliver_to_dummy): New function.
  (irc_privmsg): Try to deliver via dummy method before sending to IRC.
  (irc_notice): Likewise.
  (irc_notice_user): Likewise.
  (AddUser): If the first user mode character is '*', mark as a dummy.
  (DelUser): Unregister callback functions when destroying local clients.

src/proto-p10.c (irc_user): Bail if user is NULL or has a dummy nickname.
  (deliver_to_dummy): New function.
  (irc_notice): Try to deliver via dummy method before sending to IRC.
  (irc_notice_user): Likewise.
  (irc_privmsg): Likewise.
  (AddUser): If the first user mode character is '*', mark as a dummy.
  (DelUser): Unregister callback functions when destroying local clients.
git-archimport-id: srvx@srvx.net--2006/srvx--devo--1.3--patch-37