support optional-argument extension to getopt via double-colon

this extension is not incompatible with the standard behavior of the
function, not expensive, and avoids requiring a replacement getopt
with full GNU extensions for a few important apps including busybox's
sed with the -i option.

src/misc/getopt.c

@@ -65,9 +65,12 @@ int getopt(int argc, char * const argv[], const char *optstring)
 			}
 			return '?';
 		}
+		if (optstring[i+2] == ':') optarg = 0;
+		if (optstring[i+2] != ':' || optpos) {
 			optarg = argv[optind++] + optpos;
 			optpos = 0;
 		}
+	}
 	return c;
 }