CP-9441: debugging improvements

Option to make tapdisk not daemonise itself, this makes debugging with
valgrind much easier.

Also, documentation on how to debug tapdisk with Valgrind.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
That was not my point.
You are proposing a patch to help valgrind debugging. I do not see in the example when the option you just introduced is exploited.