Adds wasm32-unknown-emscripten as a target for nix, plus a CI job that runs the
full test suite under Node.

Source changes to compile on emscripten (which behaves like a Linux/musl libc):

* ioctl: use the Linux ioctl module and a c_int ioctl_num_type on emscripten
  (src/sys/ioctl, src/sys/mod.rs).
* resource: enable getrlimit/setrlimit and the Resource enum via libc::rlimit.
* socket: include emscripten in the SockaddrStorage AF_UNIX (pathname/unnamed)
  decode arms.
* unistd: sethostname is unavailable on emscripten, so gate it out.

The suite spawns OS threads (signal and socket-peer tests), so std is rebuilt
with atomics via -Zbuild-std and linked -pthread with -sPROXY_TO_PTHREAD so the
main thread can block; JSPI (-sJSPI) provides the return-to-host suspension for
blocking reads/writes, and NODERAWFS/NODERAWSOCKETS back the filesystem and
sockets with node's. This needs nightly + rust-src and a JSPI-capable Node
(26+, or 22 with --experimental-wasm-jspi). No custom target spec is required:
nightly now emits the __main_argc_argv entry point (rust-lang/rust#158937).

Genuinely-unsupported operations are ignored with
#[cfg_attr(target_os = "emscripten", ignore)] rather than cfg'd out, covering
fork/wait, pty/termios, mkfifo, getpwnam/setgroups/acct, flock, if_nametoindex,
pipe2 FD_CLOEXEC, AF_UNIX SCM_RIGHTS/datagram, thread signal masking, and
setrlimit; a few tests are cfg'd out where the feature is entirely absent
(raw signal actions, socketpair) or the host differs (linkat symlink follow).

Temporary, until the dependencies land upstream:

* Cargo.toml patches libc to guybedford/libc#libc-0.2-emscripten for the
  emscripten externs (rust-lang/libc#5270).
* the CI job builds against the guybedford/emscripten `cf` fork, which carries
  the NODEFS/socket/blocking patches this target depends on
  (emscripten-core/emscripten#27305, #27306).

Suite result: 148 passed, 0 failed, 35 ignored under Node - green on Linux CI.

Adds wasm32-unknown-emscripten as a target for nix, plus a CI job that runs the
full test suite under Node.

Source changes to compile on emscripten (which behaves like a Linux/musl libc):

* ioctl: use the Linux ioctl module and a c_int ioctl_num_type on emscripten
  (src/sys/ioctl, src/sys/mod.rs).
* resource: enable getrlimit/setrlimit and the Resource enum via libc::rlimit.
* socket: include emscripten in the SockaddrStorage AF_UNIX (pathname/unnamed)
  decode arms.
* unistd: sethostname is unavailable on emscripten, so gate it out.

The suite spawns OS threads (signal and socket-peer tests), so std is rebuilt
with atomics via -Zbuild-std and linked -pthread with -sPROXY_TO_PTHREAD so the
main thread can block; JSPI (-sJSPI) provides the return-to-host suspension for
blocking reads/writes, and NODERAWFS/NODERAWSOCKETS back the filesystem and
sockets with node's. This needs nightly + rust-src and a JSPI-capable Node
(26+, or 22 with --experimental-wasm-jspi). No custom target spec is required:
nightly now emits the __main_argc_argv entry point (rust-lang/rust#158937).

Unsupported operations are ignored with #[cfg_attr(target_os = "emscripten",
ignore)] rather than cfg'd out wherever they still compile, covering fork/wait,
pty/termios, mkfifo, getpwnam/setgroups/acct, flock, if_nametoindex, pipe2
FD_CLOEXEC, AF_UNIX SCM_RIGHTS/datagram, getsockname, socketpair, setrlimit, and
the linkat symlink-follow host check. The signal-delivery tests (signal, killpg,
thread mask) and the AF_ROUTE test are cfg'd out instead: compiling their
extern "C" handlers in perturbs the wasm indirect function table and aborts the
runtime.

Temporary, until the dependencies land upstream:

* the emscripten CI job injects a libc patch (guybedford/libc#libc-0.2-emscripten,
  rust-lang/libc#5270) via `cargo --config`, so other targets and CI jobs are
  unaffected.
* it also builds against the guybedford/emscripten `cf` fork, which carries the
  NODEFS/socket/blocking patches this target depends on
  (emscripten-core/emscripten#27305, #27306).

Suite result: 148 passed, 0 failed, 38 ignored under Node - green on Linux CI.