MINOR: pools: delegate parsing of command line option -dM to a new function

MINOR: pools: delegate parsing of command line option -dM to a new function

New function pool_parse_debugging() is now dedicated to parsing options
of -dM. For now it only handles the optional memory poisonning byte, but
the function may already return an informative message to be printed for
help, a warning or an error. This way we'll reuse it for the settings
that will be needed for configurable debugging options.