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>
In a if/else block, as long as one of those is multi-line, the other one needs braces regardless
of the number of its lines.