AXLink v0.2.0-alpha.2

Prerelease for the first test with radios. Tested in the lab and in CI, not yet on the
air. The test plan: BENCH.md.

What is new since v0.2.0-alpha.1:

• Every station relays by default (settings "Relay", -relay=false turns it off). A relay
  passes a beacon or CQ message on only while a station it hears directly may not have it yet,
  after a random wait in which copies from other relays count too: no relay storm where everyone
  hears everyone, no useless repeats at the end of a line.
• Packet monitor ("Monitor" / "Mithören" in the web interface): every frame heard and sent,
  those for other stations too, as with an open squelch, with a short description.
• -pcap FILE writes every frame into a file that Wireshark opens (link type AX.25, decoded
  down to the IP packets); -debug writes every frame into the log.

Lab: two stations pass all checks; three stations in a line (B in the middle) found each other
over B in all eleven runs. Stations that do not hear each other still collide at the relay at
times (one HTTP download of eleven failed); see TODO.

Packages as in alpha.1: Linux (amd64, arm64, armv6), Windows (amd64, arm64, with wintun.dll),
macOS (arm64, amd64), SHA256SUMS. Start: QUICKSTART.md.
The programs are not signed. Use your own callsign; every operator is responsible for what is
allowed on their frequency.