www/polipo: Use .conf suffix for /usr/local/etc/newsyslog.conf.d/

Update the pkg-install message to ensure the filenames mentioned in
/usr/local/etc/newsyslog.conf.d/ 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: portmgr blanket