test: allow multiple files in aresfuzz command line

If no arguments are specified, use stdin as input.
Otherwise treat each argument as a filename and feed
its contents to the fuzz entrypoint.

test/ares-fuzz.cc

@@ -1,20 +1,41 @@
 // General driver to allow command-line fuzzer (i.e. afl) to
 // fuzz the libfuzzer entrypoint.
+#include <sys/types.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <unistd.h>
 
+#include <iostream>
+
 #include <vector>
 
 extern "C" int LLVMFuzzerTestOneInput(const unsigned char *data,
                                       unsigned long size);
-int main() {
+
+static void ProcessFile(int fd) {
   std::vector<unsigned char> input;
   while (true) {
     unsigned char buffer[1024];
-    int len = read(fileno(stdin), buffer, sizeof(buffer));
+    int len = read(fd, buffer, sizeof(buffer));
     if (len <= 0) break;
     input.insert(input.end(), buffer, buffer + len);
   }
   LLVMFuzzerTestOneInput(input.data(), input.size());
+}
+
+int main(int argc, char *argv[]) {
+  if (argc == 1) {
+    ProcessFile(fileno(stdin));
+  } else {
+    for (int ii = 1; ii < argc; ++ii) {
+      int fd = open(argv[ii], O_RDONLY);
+      if (fd < 0) {
+        std::cerr << "Failed to open '" << argv[ii] << "'" << std::endl;
+        continue;
+      }
+      ProcessFile(fd);
+      close(fd);
+    }
+  }
   return 0;
 }