filemem: fix fwrite() return check in fm_write();

test `ferror()` also in `fm_err()` if `err` zero and file
NetBSD: add instructions and some workarounds (`getopt_long_only()`
  in particular)
qzint: check `__GNUC__` version for "-Wstringop-truncation"
  suppression

frontend/tests/test_args.c

@@ -1245,6 +1245,9 @@ static void test_other_opts(const testCtx *const p_ctx) {
     for (i = 0; i < data_size; i++) {
 
         if (testContinue(p_ctx, i)) continue;
+#ifdef __NetBSD__
+        if (strcmp(data[i].opt, " -bg=") == 0) continue; /* `getopt_long_only()` not available on NetBSD */
+#endif
 
         strcpy(cmd, "zint");