hexdump: Support octal and hex for -N option

GNU hexdump supports octal and hex, we add supports for BSD style
hexdump for better compatibility.

See: https://github.com/llvm/llvm-project/pull/206581/

MFC after:      2 weeks
Sponsored by:   The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58074

usr.bin/hexdump/odsyntax.c

@@ -139,7 +139,7 @@ oldsyntax(int argc, char ***argvp)
 				errx(1, "%s: invalid skip amount", optarg);
 			break;
 		case 'N':
-			if ((length = atoi(optarg)) <= 0)
+			if ((length = strtol(optarg, NULL, 0)) <= 0)
 				errx(1, "%s: invalid length", optarg);
 			break;
 		case 'O':