Add CLI options for preopened listen sockets

[haraldh]

As a follow up for #3711, this implements CLI options to insert pre-opened sockets.

--listenfd : Inherit environment variables and file descriptors following the systemd listen fd specification (UNIX only)

--tcplisten <SOCKET ADDRESS>: Grant access to the given TCP listen socket

TODO

• Write test cases

EXAMPLES

$ git clone --branch wasi https://github.com/haraldh/mio.git
$ cd mio
$ cargo +nightly build --target wasm32-wasi --example tcp_server --features="os-poll net" 
$ wasmtime run --tcplisten 127.0.0.1:9000 --env 'LISTEN_FDS=1' target/wasm32-wasi/debug/examples/tcp_server.wasm 
Using preopened socket FD 3
You can connect to the server using `nc`:
 $ nc <IP> <PORT>
You'll see our welcome message and anything you type will be printed here.
^Z
[1]+  Stopped                 wasmtime run --tcplisten 127.0.0.1:9000 --env 'LISTEN_FDS=1' target/wasm32-wasi/debug/examples/tcp_server.wasm
$ bg
$ echo Hello | nc 127.0.0.1 9000
Accepted connection from: 0.0.0.0:0
Hello world!
Received data: Hello
Connection closed
$ fg
$ ^C

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[haraldh]

rebased

[haraldh]

added mio for wasi example to first comment

[sunfishcode]

This looks like it will be a good way to enable experimentation with the new WASI accept functionality.

Looking forward, the convention of having applications implicitly assume they can get environment variables like LISTEN_FDS is problematic for WASI. To make WASI programs modular and statically analyzable, we want to make API contracts explicit in imports and exports, rather than in dynamic environment-variable conventions. I expect what we'll want in the future is for this to use the Typed Main feature, and then to have WASI libc on the inside translate that back into making getenv("LISTEN_FDS") return this information for compatibility.

However, we can address that as the pieces needed to use Typed Main come online. For now, this looks good.