[misc] Change b64_* to wrap ffmpeg's base64 encoder/decoder

Using ffmpeg's is probably better than having our own, plus it provides us with
decoded length, which we will need to support artwork via pipe.

src/httpd.c

@@ -1555,7 +1555,7 @@ httpd_basic_auth(struct evhttp_request *req, const char *user, const char *passw
 
   auth += strlen("Basic ");
 
-  authuser = b64_decode(auth);
+  authuser = (char *)b64_decode(NULL, auth);
   if (!authuser)
     {
       DPRINTF(E_LOG, L_HTTPD, "Could not decode Authentication header\n");