mail/postsrsd: revert update to 2.0.6

This reverts commit 06c06ad9fa8d2594adf5a5fe7120c623c48dd90a.

Unfortunately the /usr/local/etc/rc.d/postsrsd script does not work with
postsrsd 2.x as several command line options were dropped in favor of a
config file. Revert the update until a solution is found.

PR: 273002
Reported by: Peter Putzer <freebsd@mnd.sc>