polkitbackend: Re-instate the local authority

The local authority was removed in 0f830c76 (May 2012) as it was deemed unnecessary as the jsbackend could perform any functionality that was previously done using the localbackend.

However, for security reasons Debian and Ubuntu did not determine the JS backend safe enough to ship by default, and thus have remained on polkit version 105 ever since (though collecting cherry-picked patches from the following releases).

By re-instating the local authority both Debian and Ubuntu are able to update to track the latest version of polkit. The JS backend remains by default and the local backend is only compiled and used if configured with --disable-javascript.

While supporting two backends does come at an increased cost in this case it seems reasonable low as the code in this change required almost no modification to re-introduce.