check_smtp: send the FQDN on HELO

Workaround a check_smtp bug during which it sends a HELO with the short
hostname, spamming our exim mainlogs. Use an undocumented -F option as
described in https://bugs.debian.org/578333

Change-Id: I6c08108d60ba13f24e39fb7fce0b4ec989007f2c