Announce IRC services in the network.

Announce IRC services in the network.

This patch
 - introduces a new server flag "S" to indicate that the server can handle
   the SERVICE command (on server links),
 - implements the IRC command "SERVICE" for server-server links,
 - uses the "SERVICE" command to announce IRC services when a new
   server connects to it,
 - and fixes the Send_Message() function to let it send messages to
   services using a "target mask".

If the remote server doesn't indicate that it can handle the "SERVICE"
command (it has not set the "S" flag), services are announced as regular
users as before.