wireguard: accept all-zero private-key, public-key and preshared-key

- For PSK, an all-zero PSK means to don't do symmetric encryption. As such,
  at first it seems a bit odd when the user sets

      - preshared-key-flags != "4 (not-required)"

      - preshared-key = AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=

  Here the user indicates that a PSK is required, but then provides an
  all-zero PSK that effectively disables it. Still, we should not reject
  such a configuration. This has the benefit that it allos the user for
  being prompted for a PSK, only to disable it by entering the all-zero key.

- For the private-key (and consequently the public-key), "public-key-flags=4"
  is rejected by libnm. A private key is always required for NetworkManager to
  configure the link. However, let's not care for all-zero keys either. If the user
  configures that, we just set that key. It's a valid setting as far as WireGuard
  (the kernel module) is concerned, so we shouldn't reject it.

(cherry picked from commit 78dccb8bb9)

libnm-core/nm-utils.c

@@ -6707,11 +6707,6 @@ _nm_utils_wireguard_decode_key (const char *base64_key,
 		return FALSE;
 	}
 
-	if (nm_utils_memeqzero (bin_arr, required_key_len)) {
-		/* an all zero key is not valid either. That is used to represet an unset key */
-		return FALSE;
-	}
-
 	if (out_key)
 		memcpy (out_key, bin_arr, required_key_len);