newsyslog.conf: Restrict included files in default config to [!.]*.conf

The new default config will only include files from the following
directories which end with '.conf' and do not beginning with a '.'
character:

• /etc/newsyslog.conf.d/
• /usr/local/etc/newsyslog.conf.d/

This matches the syslog.conf(5) functionality, and also prevents '.sample' or
'.pkgnew' files being included. This is important for ports which install files
in /usr/local/etc/newsyslog.conf.d/ and also for pkgbase.

Approved by: eadler
Approved by: bapt
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D17086