Since 1.10.4 this is hard-coded to be disallowed when an activation
helper is used. That would be a security flaw waiting to happen,
and makes little sense anyway, because the activation helper sanitises
its environment.
Use BecomeMonitor() instead, as our way to assert that root and
messagebus are privileged.
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=93036
Reviewed-by: Iain Lane
Unlike the initial mitigation for CVE-2014-8148, we now allow
uid 0 to call UpdateActivationEnvironment. There's no point in root
doing that, but there's also no reason why it's particularly bad -
if an attacker is uid 0 we've already lost - and it simplifies
use of this function for future things that do want to be callable
by root, like BecomeMonitor for #46787.
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=88810
Reviewed-by: Philip Withnall
