Make xs_mkdir an xs_rm idempotent.

Make xs_mkdir an xs_rm idempotent.
When modifying libxenstore to transparently restart when the daemon dies,
it became apparent that life is simpler when all commands can simply be
restarted.  So this patch makes a slight semantic change to xs_rm and xs_mkdir:
xs_rm now succeeds if the file doesn't exist (as long as the parent exists),
and xs_mkdir succeeds if the directory already exists.
Noone should notice.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>