docs: document socket.unix and socket.serial, fix stale/missing API referencesdocs/pre-release-audit

Pre-release documentation audit against master. Adds the two fully
undocumented modules and closes gaps found by diffing every docs/*.html
page against its corresponding source:

- New docs/unix.html and docs/serial.html (socket.unix stream/dgram and
  socket.serial were never documented), linked from index.html,
  introduction.html, socket.html and reference.html. serial.html notes
  the current lack of a baud/parity/flow-control API and includes an
  os.execute+stty workaround example. Both note Windows is unsupported.
- socket.html: document headers.setcanonic, point to unix/serial modules.
- tcp.html: document getfamily, setpeername/setsockname aliases.
- udp.html: document getfamily, getfd/setfd, dirty; document the
  ipv6-multicast-hops/ipv6-unicast-hops aliasing as current (known-buggy)
  behavior rather than the originally intended semantics.
- http.html: correct redirect text (301/302/303/307/308, was 301/302
  only); document MAXHEADERLINE/MAXHEADERSIZE.
- dns.html: document getnameinfo.
- ltn12.html: document source.rewind and BLOCKSIZE.
- installation.html: fix stale "LuaSocket 3.0" sample output to 3.1.0.
- reference.html: index every anchor added above plus new Unix/Serial
  blocks.

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