Various queue-proce related updates:
- try external queue /usr/libexec/smtpd/queue-<backend> if backend is not built-in.
- allow to pass a config string to the backend.
- make queue and scheduler backends consistent with tables and filters
Merge branch 'master' into portable

Conflicts:
	smtpd/smtpd.h
Merge branch 'master' of ssh.poolp.org:/git/opensmtpd
do not prompt for key during config parsing, do it right after that
Merge branch 'master' of ssh://ssh.poolp.org/git/opensmtpd
Merge branch 'master' into portable