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.
1 job for local-authority in 2 minutes and 52 seconds (queued for 1 second)