gdbusaddress: Ignore D-Bus addresses from the environment when setuid

As with the previous commit, it’s unsafe to trust the environment when
running as setuid, as it comes from an untrusted caller. In particular,
with D-Bus, the caller could set up a fake ‘system’ bus which fed
incorrect data to this process.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #2168