Better support for default_favicon location in theme settings

Drupal themes can either provide a custom favicon_path or use the default_favicon option, and place a favicon.ico file into the theme's folder. However, the favicon module does not recognize this standard default_favicon configuration, and will still serve misc/favicon.ico, even if a favicon.ico exists in the theme folder. Currently, the only workaround is to hardcode the full path to the favicon inside the theme on the Drupal admin settings page.

The favicon module should evaluate the theme settings in a smarter way to ensure when a default favicon.ico is specified in the codebase, that it is used, regardless of whether the user specifies the path to the file in the theme settings UI or not. This will mimic the way Drupal behaves internally to produce a path to the favicon file.

This is a follow-up from #1340320: favicon not showing when set to default on theme.
Patch to follow in first comment.