Add dbus-update-activation-environment tool

If OS builders (distributions) have chosen to use the per-user bus,
this provides two possible modes of operation for compatibility with
existing X session startup hooks.

A legacy-free system can just upload DISPLAY, XAUTHORITY and possibly
DBUS_SESSION_BUS_ADDRESS into dbus-daemon's and systemd's activation
environments, similar to
http://cgit.freedesktop.org/systemd/systemd/tree/xorg/50-systemd-user.sh
installed by systemd (but unlike systemctl,
dbus-update-activation-environment works for traditional
D-Bus-activated services, not just for systemd services).

A system where compatibility is required for environment variables
exported by snippets in /etc/X11/xinit/xinitrc.d (in Red Hat derivatives,
Gentoo, etc.) or /etc/X11/Xsession.d (Debian derivatives) can upload
the entire environment of the X session, minus some selected environment
variables which are specific to a login session (notably XDG_SESSION_ID).

In Debian, I plan to put the former in a new dbus-user-session package
that enables a user-session-centric mode of operation for D-Bus,
and the latter in the existing dbus-x11 package, with the intention that
dbus-x11 eventually becomes a tool for change-averse setups or goes
away entirely.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=61301


Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>