usbconfig: use getopt(3) for option handling

usbconfig: use getopt(3) for option handling

This makes option handling consistent with other utilities as well as
Posix rules. By that, it's no longer important whether option name and
its argument are separated by a space or not, so -d5.3 works the same
as -d 5.3.

Also, recognize either /dev/ugen or ugen as prefix to the -d argument.

Note that this removes the undocumented feature that allowed to
specify multiple -d n.m options interleaved with commands referring to
that particular device in a single run.