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>
US English is the standard for programming. It is for consistency.
There are already other parts of the code where it is used. I do not think a log file with anyone picking
their favourite variant will be pleasant to see.

About the "inline" piece, my comment referred to the fact the syntax is misleading. Written this way is impossible
to distinguish among parameters for the option and explanations. If "-D" meant daemonize, how could I tell if "daemonize" is something I should type or the explanation of -D?
Unfortunately the rest of tapdisk options is inconsistent about this, so feel free to add confusion.