It's possible that 2 different /tmp/orbit-$user-$rand directories will be
created and then mis-matched;

Should be fixed by:

	<scan>
	create foo-user-random-tst
	<scan>
	rename foo-user-random-tst > foo-user-random

With a random timed unlink fallback on the 2nd <scan> if we hit another
'-tst' suffix of the right permissions.

We also need to propagate the tmpdir information from b-a-s to child by
using an ORBit2 command line parameter - in which case we can avoid the
(expensive) scan.

Also, we need some way of dealing with invisible /tmp directories. This is
going to have some appalling performance characteristic though; since we'd
need to do a well defined sequence:

/tmp/orbit-user
/tmp/orbit-user-1

   etc. until we created a directory we could use. C'est la vie. a
'security' option, that increases the likelihood of a DOS attack.