Check for duplicate arguments to tor-gencert

Found by coverity, which noticed that if you said
  tor-gencert -i identity1 -i identity2
we would leak "identity1".

[CID 1198201, 1198202, 1198203]

changes/check_dup_args_gencert

@@ -0,0 +1,3 @@
+  o Minor features:
+    - In tor-gencert, report an error if the user provides the same
+      argument more than once.

src/tools/tor-gencert.c

@@ -134,18 +134,30 @@ parse_commandline(int argc, char **argv)
         fprintf(stderr, "No argument to -i\n");
         return 1;
       }
+      if (identity_key_file) {
+        fprintf(stderr, "Duplicate values for -i\n");
+        return -1;
+      }
       identity_key_file = tor_strdup(argv[++i]);
     } else if (!strcmp(argv[i], "-s")) {
       if (i+1>=argc) {
         fprintf(stderr, "No argument to -s\n");
         return 1;
       }
+      if (signing_key_file) {
+        fprintf(stderr, "Duplicate values for -s\n");
+        return -1;
+      }
       signing_key_file = tor_strdup(argv[++i]);
     } else if (!strcmp(argv[i], "-c")) {
       if (i+1>=argc) {
         fprintf(stderr, "No argument to -c\n");
         return 1;
       }
+      if (certificate_file) {
+        fprintf(stderr, "Duplicate values for -c\n");
+        return -1;
+      }
       certificate_file = tor_strdup(argv[++i]);
     } else if (!strcmp(argv[i], "-m")) {
       if (i+1>=argc) {