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.h

@@ -116,7 +116,7 @@ void run_benchmark(char *name, void (*benchmark)(void*, int), void (*setup)(void
 int have_flag(int argc, char** argv, char *flag) {
     char** argm = argv + argc;
     argv++;
-    while (argv != NULL && argv != argm) {
+    while (argv != argm) {
         if (strcmp(*argv, flag) == 0) {
             return 1;
         }
@@ -125,6 +125,32 @@ int have_flag(int argc, char** argv, char *flag) {
     return 0;
 }
 
+/* takes an array containing the arguments that the user is allowed to enter on the command-line
+   returns:
+      - 1 if the user entered an invalid argument
+      - 0 if all the user entered arguments are valid */
+int have_invalid_args(int argc, char** argv, char** valid_args, size_t n) {
+    size_t i;
+    int found_valid;
+    char** argm = argv + argc;
+    argv++;
+
+    while (argv != argm) {
+        found_valid = 0;
+        for (i = 0; i < n; i++) {
+            if (strcmp(*argv, valid_args[i]) == 0) {
+                found_valid = 1; /* user entered a valid arg from the list */
+                break;
+            }
+        }
+        if (found_valid == 0) {
+            return 1; /* invalid arg found */
+        }
+        argv++;
+    }
+    return 0;
+}
+
 int get_iters(int default_iters) {
     char* env = getenv("SECP256K1_BENCH_ITERS");
     if (env) {