rc.d/sendmail: Return non-zero if the daemon fails to start or is not running

If you have a mail server that is running sendmail daemon
(sendmail_enable=YES) and sendmail queue runner (sendmail_msp_queue=YES)
and the sendmail daemon dies, /etc/rc.d/sendmail status does see the
daemon is not running but returns 0 as the exit code. This prevents
other programs (like puppet) from restarting sendmail to fix the issue.

Make sure that the exit code is propagated towards the end of the script
if any of the sendmail services fail.

This patch does not call exit directly but instead just sets the exit
status code by calling exit in a subshell. This way we do not exit the
current shell in case the service script is sourced (e.g., when
rc_fast_and_loose is active).

PR: 223132
MFC after: 2 weeks
Reported by: pirzyk
Discussed with: jilles, eugen
Reviewed by: christos, gshapiro (previous version), markj
Approved by: christos (mentor), markj (mentor)
Differential Revision: https://reviews.freebsd.org/D46862
Co-authored-by: Jim Pirzyk <pirzyk@FreeBSD.org>