Option to skip disabled modules

In some cases it's not needed to check the status of disabled modules, as it will give a status warning for modules not in use.
For example, this is the case when using installation profiles with loads of modules, but only few in use.

Here's a patch that adds the option to disable the update check for disabled modules in the prod_check settings form.
I added two hooks to force checking updates for disabled modules as in some cases modules are used, but not enabled in drupal, f.e. cache or database backends (apc, memcache, varnish, mongodb, ...) etc. These modules need to be checked as well.