Simplify drupal_system_listing() parameters

As determined in #1808200-15: Unit test performance significantly decreased since system list config conversion:

change the entire function signature; first and foremost to remove the DRUPAL_PHP_FUNCTION_PATTERN from all callers and just have $dir and $extension, and only optionally $min_depth and $key in there

i.e., the resulting signature should look like this:

# The 99.9% use-case:
$modules = drupal_system_listing('modules');
$themes = drupal_system_listing('themes');
$profiles = drupal_system_listing('profiles');

# But still allow to override default parameters:
$modules = drupal_system_listing('modules', '.module', 0, 'uri');

$libraries = drupal_system_listing('libraries', '.info', 1, 'filename');

Additionally, consider to add a generic helper function to bootstrap.inc that returns a simple mapping array for core-supported directory names and file extensions — since this mapping is apparently repeated in drupal_get_filename().