parseopt: add PARSE_OPT_NODASH

Add support for options that don't start with a dash.  Initially, they
don't accept arguments and can only be short options, i.e. consist of a
single character.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-parse-options.c

@@ -54,6 +54,8 @@ int main(int argc, const char **argv)
 		OPT_ARGUMENT("quux", "means --quux"),
 		OPT_NUMBER_CALLBACK(&integer, "set integer to NUM",
 			number_callback),
+		{ OPTION_BOOLEAN, '+', NULL, &boolean, NULL, "same as -b",
+		  PARSE_OPT_NOARG | PARSE_OPT_NONEG | PARSE_OPT_NODASH },
 		OPT_GROUP("Standard options"),
 		OPT__ABBREV(&abbrev),
 		OPT__VERBOSE(&verbose),