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>
1. Even if it is one statement, it is 2 lines long, so use braces. It is legal but unpleasant
2. This is an improvement not related to this fix, so it should be at least in a different commit and the PR should be more generic