[WIP] Rewrite m_helpop to be compatible with other server implementations.

## Major Changes
1. We now use the same numerics as the ircd-ratbox 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 &lt;helpop:title&gt; and the help message has been moved to
   &lt;helpop:message&gt;.
3. &lt;helpop key="nohelp"&gt; has been moved to &lt;nohelp:missingmsg&gt;.
4. Help messages can now depend on modules being loaded. The message
   shown when a module is not loaded can be customised using
   &lt;nohelp:modulemsg&gt;.

## TODO

1. Make '/HELPOP INDEX' sort into columns to save space.
2. Possibly make the '/HELPOP' page customisable.
3. Rewrite helpop.conf.example and helpop-full.conf.example in
   the new format.