Rewrite m_helpop to be compatible with other server implementations.

1. We now use the same numerics as the IRCD-Hybrid family for help
   instead of incorrectly using the numerics used by UnrealIRCd.
2. The title in the first line of the old HELPOP response has been
   moved to <helpop:title> and the help message has been moved to
   <helpop:message>.
3. <helpop key="nohelp"> has been moved to <helpmsg:missingtopic>.
4. Help messages can now depend on modules being loaded. The message
   shown when a module is not loaded can be customised using
   <helpmsg:missingmodule>.