convert "no-" command line option prefix into "=no" for OPT_CUSTOM

convert "no-" command line option prefix into "=no" for OPT_CUSTOM

... to allow restoring/retaining previous behavior for options getting
converted from boolean to custom. Obviously that'll work only when no
other argument was specified for the option. Command line settings of
the form "no-<name>=<value>" will now be ignored as ambiguous (rather
than being interpreted as "<name>=<value>", i.e. ignoring the "no-"
prefix).

Reported-by: Don Slutz <dslutz@verizon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>