dbus-daemon(1): Mention and deprecate shared session buses

dbus-daemon(1): Mention and deprecate shared session buses

This might (?) have made sense behind a firewall in 2003; but now it's
2018, the typical threat model that we are defending against has
changed from "vandals want to feel proud of their l33t skills"
to "organised crime wants your money", and a "trusted" local LAN
probably contains an obsolete phone, tablet, games console or
Internet-of-Things-enabled toaster with remote root exploits.
This make network topologies that used to be acceptable look
increasingly irresponsible.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=106004
Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
(cherry picked from commit d0a16b59a8572fbd1934e941e2e3004840306222)