send-email: allow sendmail binary to be used instead of SMTP

This should make local mailing possible for machines without
a connection to an SMTP server.

It'll default to using /usr/sbin/sendmail or /usr/lib/sendmail
if no SMTP server is specified (the default).  If it can't find
either of those paths, it'll fall back to connecting to an SMTP
server on localhost.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>