opt: add OPT_EARLY and opt_early_parse.

opt: add OPT_EARLY and opt_early_parse.

Parsing options like --verbose and --debug can be a pain.  You need to
have everything set up before invoking parse_args(), but that may be a
significant amount of work, for which you may want verbose or
debugging enabled.

Thus the concept of "early" args: you can nominate arguments to be
parse before anything else, using opt_early_parse().