```
net-mgmt/nagios: Adding monplugins flavor to nagios
Some users asked to add a flavor to permit to have nagios already
packaged with monplugins.
This patch add this functionality and has been tested (both flavors
default and monplugins) with poudriere under 13.4-RELEASE.
PR: 283977
Sponsored by: Klara, Inc.
Approved by: 0mp (mentor)
```