Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Merge from OpenBSD:
o Realloc memory leak fixed which won't matter but would trigger purify
o Default to sendmail when no mailer.conf exists.

Fixed bugs in OpenBSD version:
o Add NULL termination in the right place.

Also put back the err. free shouldn't touch errno.

Pointed out by: theo de raadt (except the NULL bug :-)

Move mailer.conf to /etc/mail at obrien's request.

Make mailwrapper build