opt: add new parse_early_args_incomplete.

opt: add new parse_early_args_incomplete.

If we have plugins, and those can register args, we have a problem finding
the plugin dir!  So, do a best-effort incomplete parse.

Note that this can screw up in theory if we have "--unknown --foo" since we
don't know if unknown takes an argument (in which case, ignore --foo) or
not.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>