dbus-launch: add --exit-with-x11 option

dbus-launch: add --exit-with-x11 option

This is more suitable for distributions' Xsession scripts: it verifies
that X is already available, and so never results in an attempt to poll
stdin.

We read the machine UUID because it is needed to set the X atoms.
x11_init() assumes that the machine UUID (global variable) has been
set, either via read_machine_uuid_if_needed() or save_machine_uuid().
This is pretty tangled, but to make The Right Thing happen
automatically, we'd need to redo dbus-launch in terms of DBusError.

Reviewed-by: Will Thompson
Reviewed-by: Thiago Macieira
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=39197
Signed-off-by: Simon McVittie <smcv@debian.org>