mail/heirloom-mailx: Use the proper progname in argv[0]

When argv[0] does not match that af the program name passed to
execv(2), mailwrapper cannot determine which application to call
in its translation table resulting in:

mailwrapper: no mapping in /etc/mail/mailer.conf

(cherry picked from commit 849299d0d6cc566e3cf9681755dbcfdb8c38b9a3)