Backport of drush nagios-updates command to D6

While D6 won't be supported for that much longer, backporting the drush nagios-updates command seems pretty trivial.

I had a look at the system_modules form in D6 but that is structured very differently to the D7 version, so getting the 'ignore from nagios' checkboxes to appear would be quite involved. For a stop-gap solution, modules/themes to ignore could just be specified in settings.php.

Here's the list of available updates on my D6 test install:

$ drush nagios-up all
ctools pathauto

$ drush nagios-up security
ctools

Ignore the pathauto update:

$ php -r "print json_encode(array('pathauto' => TRUE));" | drush vset --format=json nagios_ignored_modules -
nagios_ignored_modules was set to
array (
  'pathauto' => true,
).

$ drush nagios-up all
ctools

$ drush nagios-up security
ctools

Ignore the ctools security update:

$ php -r "print json_encode(array('ctools' => TRUE));" | drush vset --format=json nagios_ignored_modules -
nagios_ignored_modules was set to
array (
  'ctools' => true,
).

$ drush nagios-up all
pathauto

$ drush nagios-up security