* Adds ipv6 support. Compiles with ipv6 enabled and then accepts ipv6
connections and answers AAAA questions.
* Fixes a few overflows of the reply buffer. Reduce the default reply buffer
size, name it MDNS_RESPONDER_REPLY_SIZE, and allow overriding.
* Fix mdns_add_TXT.
* Prefer malloc to large stack buffers, to control stack sizes, and try to
malloc only the buffer size needed where known in advance.
* Determine the IP addresses when responding, eliminating the update function
and the update task.
* Allow use in SoftAP and StationAP mode too.
* Fix to compile without the debug output.
* Slightly better integration with lwip.
* Accept a NULL TXT entry.
* Some code style changes, not comprehensive.
