options: allow user to specify an alternative to getopt.gnu_getopt.

options: allow user to specify an alternative to getopt.gnu_getopt.

The most likely alternative is getopt.getopt, which doesn't rearrange
arguments.  That would mean "-a foo -p" is considered as the option "-a"
followed by the non-option arguments ['foo', '-p'].

The non-gnu behaviour is annoying most of the time, but can be useful when
you're receiving command lines that you want to pass verbatim to someone
else.