mail/mailman: Use .conf suffix for /usr/local/etc/newsyslog.conf.d/

• Rename the files installed to /usr/local/etc/newsyslog.conf.d/ to end with a '.conf' suffix.
• Add pkg-install script to automatically move any copies of the old newsyslog file to the new location if the new file is unmodified from the default, or print a warning if it the new file has been modified.
• Add a note to UPDATING and pkg-message to warn users of this, in case they are using provisioning/configuration management tools which need to be modified. Note the UPDATING entry was committed in r485721.

Recent changes to /etc/newsyslog.conf (r340318) will only include files
from the /usr/local/etc/newsyslog.conf.d/ directory which end with
'.conf' and do not beginning with a '.' character.

Reviewed by: mat
Approved by: mandree (maintainer)
Differential Revision: https://reviews.freebsd.org/D17088