Make uid 0 immune to pending_fd_timeout limit

Make uid 0 immune to pending_fd_timeout limit

This is a workaround for
<https://bugs.freedesktop.org/show_bug.cgi?id=95263>. If a service
sends a file descriptor sufficiently frequently that its queue of
messages never goes down to 0 fds pending, then it will eventually be
disconnected. logind is one such service.

We do not currently have a good solution for this: the proposed
patches either don't work, or reintroduce a denial of service
security vulnerability (CVE-2014-3637). Neither seems desirable.
However, we can avoid the worst symptoms by trusting uid 0 not to be
malicious.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=95263
Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1591411
Reviewed-by:  Łukasz Zemczak
Tested-by: Ivan Kozik
Tested-by: Finn Herpich
Tested-by: autostatic
Tested-by: Ben Parafina
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>