Simple mDNS + DNS-SD impl

Main TODOs:
1. the spec requires the impl to resend the reply once and
   to send gratuitous updates

2. advertise our clubby services as DNS-SD sub-services.

---

Tested with:

1. https://play.google.com/store/apps/details?id=com.grokkt.android.bonjour&hl=en
2. https://play.google.com/store/apps/details?id=com.druk.bonjour.browser&hl=en
3. osx `dns-sd`

Works well enough; it doesn't have some glitches due the fact that we
don't retransmit yet and we don't send gratuitous updates.

PUBLISHED_FROM=38d7a50beb274d4aeeda5b0b58e44dfa0789da84

docs/c-api/dns.h/intro.md

@@ -8,6 +8,7 @@ items:
   - { name: mg_dns_insert_header.md }
   - { name: mg_dns_copy_questions.md }
   - { name: mg_dns_encode_record.md }
+  - { name: mg_dns_encode_name.md }
   - { name: mg_parse_dns.md }
   - { name: mg_dns_uncompress_name.md }
   - { name: mg_set_protocol_dns.md }