bench: add --help option to bench.

The following functions were created:
    1. bench.c: help()
        - prints the help to the command-line
    2. bench.h: have_invalid_args()
        - takes a list of arguments that the user is allowed to enter on the command-line
        - returns 1 if the user entered an invalid argument
        - returns 0 if all the user entered arguments are valid

src/bench_ecmult.c

@@ -292,7 +292,7 @@ int main(int argc, char **argv) {
            || have_flag(argc, argv, "--help")
            || have_flag(argc, argv, "help")) {
             help(argv);
-            return 1;
+            return 0;
         } else if(have_flag(argc, argv, "pippenger_wnaf")) {
             printf("Using pippenger_wnaf:\n");
             data.ecmult_multi = secp256k1_ecmult_pippenger_batch_single;