Replace the $rcexec variable by an rc_exec function.

Replace the $rcexec variable by an rc_exec function.
It is much cleaner to the eyes and makes more sense from a functionnal point of
view.
This will allow to extend rc_exec with other functionnalities (like upcoming
rc_startdir).
Bonus point: daemon_logger will now work with manually crafted rc_start
functions.

This will require a mechanical change from ${rcexec} to rc_exec in rc.d scripts.
ports will be fixed right after this commit but we will keep compatibility to
give a chance to people to fix their custom scripts.

positive tests from a few
ok robert@