The titlebar with "Mozilla Firefox" has been fixed but there
are some more bits that can be cleaned up.

This commit achieves that by:

1. Make sure more of the environment is bubbled through anaconda to
   the webui launcher. In particular, we need XDG_CURRENT_DESKTOP, but
   this commit brings it all through, so firefox runs in an environment
   as close to getting run directly by the live user as possible.
2. Two exceptions are XAUTHORITY and XDG_RUNTIME_DIR which need to
   remain unset until we can run firefox as a normal user instead of root.

At the moment most of the firefox command line is getting placed
in a variable named $BROWSER and then getting run as

$BROWSER http://url

This only works if $BROWSER is at the very front of the line or if
it's run through eval.

Instead, make BROWSER into an array so it's positional arguments
get expanded positionally.

It's not a good idea to run UI code as root if we can help it, and
since the webui separates front end from backend, we don't need to
run the front end code as root.

This commit changes webui-desktop to start firefox as the liveuser.

The entire script could probably be run unprivileged with a few
changes to the cockpit parts (different port, new polkit policy,
cockpit.spawn changes to run as superuser), but that's a change
for another time.