[DX] Introduce a new `user_permission_get_info()` function

Modules can specify permissions via hook_permission():

function my_awesome_module_permission() {
  return array(
    'permission x' => array(
      'title' => t('Permission to do x'),
      'description' => t('Allows people to do x.'),
      'restrict access' => TRUE,
      'warning' => t('Allows people to do ...'),
    ),
    'permission y' => array(
      'title' => t('Permission to do y'),
      'description' => t('Allows people to do y.'),
    ),
  );
}

Now, we have user_access(), which allows us to determine if a user has a specific permission:

$user_can_do_x => user_access('permission x');
$user_can_do_y => user_access('permission y');

...and we also have user_permission_get_modules(), which returns all permissions, along with which module implements each permission.

...but we don't have any function that returns the properties of a permission. So each time we need to reference a permission say in some help text, we do this:

$form['whatever'] = array(
  ...
  '#description' => t('This option depends on the "Permission to do y" permission.',
  ...
);

This is OK, but if at some point we rename the permission (change it's 'title'), then we need to do a search and replace (at best), in order to find all instances of the literal permission title in the codebase, and update it to the new name/label. Ideally though, we should be able to do something like this:

$permission_info = user_permission_get_info('permission x');
$form['whatever'] = array(
  ...
  '#description' => t('This option depends on the @permission permission.', array('@permission' => $permission_info['title'],
  ...
);

That way, we would only need to update any permission info in a single place (in its hook_permission() function), which also makes translators' life easier.

As a developer, at minimum I would like this new function to return:

• the title of the permission
• the description of the permission

...additional nice-to-have's:

• an admin path, which can be used to provide a link to /admin/config/people/permissions#permission x (scrolls the permissions listing page to the specific permission)
• the module that specifies the hook of the permission (alternatively, have user_permission_get_modules() accept an optional $permission parameter, so to be able to do user_permission_get_modules('permission x')). This could also be used as a search/filter query when #980 gets implemented
• a list of all user roles this permission has been assigned to (we have user_roles() for that - still nice to have)