massively simplify run-with-tmp-session-bus.sh by using dbus-run-session

massively simplify run-with-tmp-session-bus.sh by using dbus-run-session

It turns out that if you don't second-guess the system by catching
SIGINT, the right things happen: it's received by every program in the
foreground process group, including dbus-run-session and dbus-daemon.
Neither of those catch SIGINT (unlike dbus-launch) so they'll exit
gracefully without the wrapper script needing to do anything special.

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