net-mgmt/nagios: Add monplugins flavor

Some users asked to add a flavor to permit to have nagios already
packaged with monplugins.

PR:	283977
Sponsored by:	Klara, Inc.
Approved by:	0mp (mentor)
Approved by:	joneum (maintainer)
Differential Revision:	https://reviews.freebsd.org/D48146