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>
If the option is not mandatory, as I think, square brackets have to be used.
Moreover, even if I saw another piece of code where "inline" explanations of options are present, it is not very
common and I would use, at least a way to show it is a piece of doc and not a parameter for the option.