Replace "Nagios::Plugin" with "Monitoring::Plugin"

As this basically is an upstream renaming of the package to avoid
trademark issues, drop-in substitution is satisfactory.

Closes Flameeyes/nagios-plugins-flameeyes#1

Bug: https://bugs.gentoo.org/608224

check_openrc.pl

@@ -36,7 +36,7 @@ =head1 LICENSE
 =cut
 
 use strict;
-use Nagios::Plugin;
+use Monitoring::Plugin;
 
 my $LICENSE = <<END;
 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
@@ -48,7 +48,7 @@ =head1 LICENSE
 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 END
 
-my $np = Nagios::Plugin->new( shortname => "OPENRC",
+my $np = Monitoring::Plugin->new( shortname => "OPENRC",
 			      usage => "Usage: %s [--stopped-critical] <list of runlevels>",
 			      blurb => "Check services status on OpenRC",
 			      license => $LICENSE,

check_portage_age.pl

@@ -37,7 +37,7 @@ =head1 LICENSE
 =cut
 
 use strict;
-use Nagios::Plugin;
+use Monitoring::Plugin;
 use Date::Parse;
 use Time::Duration;
 
@@ -51,7 +51,7 @@ =head1 LICENSE
 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 END
 
-my $np = Nagios::Plugin->new( shortname => "PORTAGE_AGE",
+my $np = Monitoring::Plugin->new( shortname => "PORTAGE_AGE",
 			      usage => "Usage: %s -w SECONDS -c SECONDS",
 			      blurb => "Check Portage tree age (last sync)",
 			      license => $LICENSE,