scripts: tracebd.py/dbgbmap.py: Made off range a subparser of block range

So:

- before: ./scripts/dbgbmap.py disk -b4096 -@0 -n16,32
- after:  ./scripts/dbgbmap.py disk -b4096 -@'0 -n16,32'

This is mainly to avoid the naming conflict between -n/--size and
-n/--lines, while also separating out the namespaces a bit.

It's probably not the most intuitive CLI UI, but --off and -n/--size are
probably infrequent arguments at this level of script anyways.

scripts/plotmpl.py

@@ -1479,6 +1479,7 @@ if __name__ == "__main__":
         def parse(value):
             import copy
             subparser = copy.deepcopy(parser)
+            subparser.prog = "%s --subplot" % parser.prog
             next(a for a in subparser._actions
                     if '--output' in a.option_strings).required = False
             next(a for a in subparser._actions