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/mg_dns_encode_name.md

@@ -0,0 +1,10 @@
+---
+title: "mg_dns_encode_name()"
+decl_name: "mg_dns_encode_name"
+symbol_kind: "func"
+signature: |
+  int mg_dns_encode_name(struct mbuf *io, const char *name, size_t len);
+---
+
+Encodes a DNS name. 
+