Support blocking socket ops via a single fd_wait primitive - #27342

Blocking accept/recv on sockets, replacing the previous approach of marking
the socket data syscalls __async (which, under JSPI, wrapped every one of
them in WebAssembly.Suspending - taxing every nonblocking call with a
suspend/resume round-trip, since a Suspending import always resolves through
a Promise in V8).

The data syscalls stay strictly synchronous imports: single attempt, -EAGAIN
when they would block. Blocking is factored into one new import,
_emscripten_fd_wait(fd, events), and retry loops in the musl wrappers
(accept, accept4, recvfrom, recvmsg): on EAGAIN with a blocking fd and no
MSG_DONTWAIT, wait for readiness on the inode's listener queue and retry.

This is a pthreads-only facility. The retry loops compile only into the -mt
libc (gated on __EMSCRIPTEN_PTHREADS__ - the only compile-time boundary libc
has; ASYNCIFY is a link-time transform with no libc variant), and
_emscripten_fd_wait blocks only on a proxied pthread worker: __proxy sync +
__async gives the PROXY_SYNC_ASYNC call path, whose sync-proxy completes -
ending the worker's futex wait - when the returned Promise resolves. In every
other context, including the event-loop thread which cannot block, it fails
with -EAGAIN. Single-threaded ASYNCIFY/JSPI builds use epoll for readiness
instead, so a purely-synchronous build keeps the direct doReadv/doWritev path
byte-for-byte and hello-world code size is unchanged.

accept4 now applies SOCK_NONBLOCK to the accepted fd (on top of the flags it
inherits from the listener); without this a SOCK_NONBLOCK accept off a
blocking listener wrongly yielded a blocking socket.

Send/write paths are untouched: the node backend buffers and never
would-blocks, so blocking send degenerates to synchronous buffered success
and needs no wait machinery. read()/write() on a socket fd are likewise not
covered - only the socket calls themselves.

Tested with test_noderawsockets_tcp_blocking (blocking accept + recv that must
suspend, under PROXY_TO_PTHREAD) and test_noderawsockets_tcp_accept_nonblock
(accept4 SOCK_NONBLOCK off a blocking listener), plus the mio test suite under
PROXY_TO_PTHREAD + NODERAWSOCKETS + NODERAWFS: 144 passed, 0 failed, 5 ignored.