Make the bus launcher abort if an instance is already running

Upon starting up, at-spi-bus-launcher now looks for an X property with
the bus address and tries to connect to it, aborting if successful.
This fixes a bug where, if the launcher was run twice (perhaps by both
an autostart script and a request for the bus address), the second
instance would start its main loop, eventually abort, and delete the X
property, which would cause a new X property to be created later,
overriding the old one, if a different user requested the address,
leading to a11y breakage.