Add missing label key to named Getopts args

If the `label` key is given as a named argument for argument
specification, if it turns out to be the first element returned in the
argument list subject to hash randomization, then this test for named
arguments fails due to the omission of the label from this alternating
group in the pattern.

When this happens, the code for an unnamed list of items was run
instead, leading to a very confusing area happening randomly:

    8 parameters were passed to Monitoring::Plugin::Getopt::arg but 2 - 5 were expected
     at PERL/lib/perl5/Monitoring/Plugin/Getopt.pm line 397.
            Monitoring::Plugin::Getopt::arg(undef, "label", "HOSTNAME", "required", 1, "help", "Hostname of device to check", "spec", ...) called at PERL/lib/perl5/Monitoring/Plugin.pm line 161
            Monitoring::Plugin::add_arg(Monitoring::Plugin=HASH(0x1f90fd8), "label", "HOSTNAME", "required", 1, "help", "Hostname of device to check", "spec", ...) called at libexec/check_example line 144

If you specified all five keys for your argument, then this happens
(roughly) one-fifth of the time.

Author: tejr

lib/Monitoring/Plugin/Getopt.pm

@@ -383,7 +383,7 @@ sub arg
   my %args;
 
   # Named args
-  if ($_[0] =~ m/^(spec|help|required|default)$/ && scalar(@_) % 2 == 0) {
+  if ($_[0] =~ m/^(spec|help|required|default|label)$/ && scalar(@_) % 2 == 0) {
     %args = validate( @_, {
       spec => 1,
       help => 1,