Add Postfix XCLIENT support to smtp-forward plugin

manually merged in PR #2 from cventers

XCLIENT support allows Qpsmtpd to forward client information, such as
the IP address and HELO information, to Postfix such that it can use
that information in access control decisions and logging.

XCLIENT is documented here: http://www.postfix.org/XCLIENT_README.html

This patch adds a "xclient" argument to smtp-forward which enables the
use of the XCLIENT verb if it is advertised by the server smtp-forward
is delivering mail to.