Host netdev changes from a large program running under TinyGo: which would you take? #101

These host netdev changes come from running a large networked program (skywire) natively under TinyGo. Bug fixes are filed separately (#98, #99, #100), and some gaps already have issues (#93, #95, #97). These are larger and most build on #77, so I would like to know which you want before preparing PRs.

• Unconnected UDP. ReadFrom, WriteTo, ReadMsgUDP and WriteMsgUDP return "not implemented", so a UDP server cannot learn who sent a packet. 324ec10
• Interfaces, InterfaceAddrs and the AddrPort UDP message calls, which ICE agents such as pion need. d36490a
• Unix domain stream sockets, including SCM_RIGHTS through ReadMsgUnix and WriteMsgUnix. godbus needs these. 664ed05
• SyscallConn on connections and listeners, and ListenConfig with Control (net: SyscallConn, Dialer.Control and IPConn.Read are missing for grpc and wazero #95). 7820a37
• FileConn, FileListener and File (net: FileListener, FileConn and TCPListener.File are missing #93), for systemd socket activation and passed descriptors. 58f7e89
• Let the kernel pick port 0 for UDP. A per process counter from 49152 collides between two TinyGo programs on one host. c987a68
• Cache /etc/hosts and resolv.conf. With a 6.5 MB ad block hosts file each lookup allocated 26 MB. a95650b
• On top of net: non-blocking host netdev I/O with an epoll poller #77, keep an fd's poll state between waits and reuse its waiter channel and timer. Each blocking read made about 1 KB of garbage and two epoll_ctl calls. 27e5edb a8b98cd