Unix: delete any (empty) directories we created on exit

If the program shuts down cleanly then the empty directories will
also be removed. If the program fails to close event channels or
unmap memory then the unlink will deliberately fail to highlight
the failure.

Signed-off-by: David Scott <dave.scott@citrix.com>
Unix: print "FOO=\"\"; export FOO" for easier cut 'n paste/eval

The output now looks the same (to me) as "opam config env".

Signed-off-by: David Scott <dave.scott@citrix.com>