stdio-bridge: add support for --system and --user

stdio-bridge: add support for --system and --user

So far, the bridge always acted as if "--system" was used, i.e. would
unconditionally connect to the system bus. Let's add "--user" too, to
connect to the users session bus.

This is mostly for completeness' sake.

I wanted to use this when making sd-bus's ability to connect to other
user's D-Bus busses work, but it didn't exist so far. In the interest of
keeping things compatible the implementation in sd-bus will not use the
new "--user" switch, and instead manually construct the right bus path
via "--path=", but we still should add the proper switches, as
preparation for a brighter future, one day.