Improve Illumos packaging, docs, support

[ryancnelson]

Describe the bug
at https://github.com/tailscale/tailscale/wiki/Hosts , Brad asks us to file bugs to add to that list.
This bug is me, doing that.

To Reproduce
Steps to reproduce the behavior:
see https://blog.shalman.org/tailscale-for-illumos/ for initial work to make this work.

Additional context
If Illumos testing infrastructure is required to do builds and run interop tests with other hosts and OS's running Tailscale, I can volunteer my homelab in Seattle.

[bradfitz]

I've fleshed out https://github.com/tailscale/tailscale/wiki/Hosts and added Illumos to that list.

Will keep this bug open about general Illumos improvements.

[DentonGentry]

Note also #697 about packaging specifically, which we'll close when the package makes it upstream in WireGuard/wireguard-go#39.

WireGuard/wireguard-go#39 was closed without merging in 3/2023.