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 }

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. 
+

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

@@ -15,11 +15,11 @@ are taken from the parameters, encoded in the appropriate format depending on
 record type and stored in the IO buffer. The encoded values might contain
 offsets within the IO buffer. It's thus important that the IO buffer doesn't
 get trimmed while a sequence of records are encoded while preparing a DNS
-*reply.
+reply.
 
 This function doesn't update the `name` and `rdata` pointers in the `rr`
-*struct
-because they might be invalidated as soon as the IO buffer grows again.
+struct because they might be invalidated as soon as the IO buffer grows
+again.
 
 Returns the number of bytes appened or -1 in case of error.