net-mgmt/icinga2: Use .conf suffix for /usr/local/etc/newsyslog.conf.d/

Rename files intended for installation 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: lme (maintainer)
Differential Revision: https://reviews.freebsd.org/D17091