Handle activated child stdout/stderr correctly in systemd environment

Handle activated child stdout/stderr correctly in systemd environment

In systemd environment, dbus-daemon will run as no-fork mode since this
is the recommended practice of systemd.

In that scenario, child activated by dbus-daemon will inherit
dbus-daemon standard streams, includes stdin/stdout/stderr. stdin will
be redirected to /dev/null by systemd and stdout/stderr will be catched
by systemd log subsystem. Since the child inherit stdout/stderr from
dbus-daemon, so from systemd journal log, the child log output will be
identified with dbus-daemon identifier. So it's a little confusing.

This patch redirects the child stdout/stderr to systemd journal stream,
and with its owned service name as identifier. However, thing not fixed
perfectly due to the socket ucred of the child is owned by dbus-daemon,
so the pid isn't the real pid of the chile.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=68559
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>