net-mgmt/nagios-check_clamav: add NAGPLUGINS option

Add NAGPLUGINS option to choose between monitoring-plugins and
nagios-plugins.
Update MASTER_SITES and WWW.
Add LICENSE and LICENSE_FILE. The file is extracted from source.
Add NO_ARCH=yes.
Switch to DISTVERSION.
Pet portfmt.

PR: 255031
Approved by: maintainer timeout (2+ weeks)