Organize queue_proc.c code as other queue backends.
update the queue backend API to only define an init call and get rid of the
QOP_* defines.  Builtin backends use the same API as external ones to
register their callback.
Remove sc_pid from struct smtpd. It's not used anywhere.
Merge commit 'd80bff90213aa1a7dc29c05e4b1ba041d94b6e83'
document error:
Merge branch 'master' of https://github.com/poolpOrg/OpenSMTPD