cpio: if longopts are enabled, accept --null (synonym for -0)

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

archival/cpio.c

@@ -376,6 +376,7 @@ int cpio_main(int argc UNUSED_PARAM, char **argv)
 #endif
 		"owner\0"        Required_argument "R"
 		"verbose\0"      No_argument       "v"
+		"null\0"         No_argument       "0"
 		"quiet\0"        No_argument       "\xff"
 		"to-stdout\0"    No_argument       "\xfe"
 		;