Fix some mingw build warnings

These include:
   - Having a weird in_addr that can't be initialized with {0}
   - Needing INVALID_HANDLE_VALUE instead of -1 for file handles.
   - Having a weird dependent definition for struct stat.
   - pid is signed, not unsigned.

src/test/test.c

@@ -72,7 +72,7 @@ int have_failed = 0;
  * our files during testing. */
 static char temp_dir[256];
 #ifdef _WIN32
-#define pid_t unsigned
+#define pid_t int
 #endif
 static pid_t temp_dir_setup_in_pid = 0;
 

src/test/test_util.c

@@ -2274,8 +2274,13 @@ run_util_spawn_background(const char *argv[], const char *expected_out,
   test_assert(process_handle != NULL);
   test_eq(expected_status, process_handle->status);
 
+#ifdef _WIN32
+  test_assert(process_handle->stdout_pipe != INVALID_HANDLE_VALUE);
+  test_assert(process_handle->stderr_pipe != INVALID_HANDLE_VALUE);
+#else
   test_assert(process_handle->stdout_pipe > 0);
   test_assert(process_handle->stderr_pipe > 0);
+#endif
 
   /* Check stdout */
   pos = tor_read_all_from_process_stdout(process_handle, stdout_buf,