sysutils/munin-*: 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 it was modified from the default or delete the old file if it is has not 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.

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: mandree (D17088)
Approved by: mat (maintainer)
Differential Revision: https://reviews.freebsd.org/D17089