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.

Proposed changes to /etc/newsyslog.conf will only include files from the
/usr/local/etc/newsyslog.conf.d/ directory which end with '.conf' and do
not beginning with a '.' character. https://reviews.freebsd.org/D17086

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