Export dbus_setenv() as a utility function

Export dbus_setenv() as a utility function

It's sufficiently portable that GLib has an equivalent, and I really
don't want to have to either open-code it in dbus-run-session or
link dbus-run-session statically. We have enough statically-linked
rubbish already.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=39196
Reviewed-by: Colin Walters <walters@verbum.org>