[PHP] Support non-blocking read streams

[adamziel]

Description

This PR adds supports for reading from non-blocking streams
(stream_set_blocking($fp, true)) by aligning the streaming logic with
unix kernel:

• stream_set_blocking() is now able to correctly mark the OS stream as
  non blocking via fcntl()
• fd_read() either blocks or immediately returns based on stream type
• js_open_process(), proc_open() et al. now work directly with
  kernel pipes. There's no more special event-based handling, polling, or
  special casing for pumping stdin data to the process.
• PIPEFS no longer returns EWOULDBLOCK when the pipe is blocking, lacks
  data, and the other side is already closed
• Removed special cases for polling processes, including
  PHPWASM.child_proc_by_fd. We rely on kernel pipes now.
• Removed PHP 7.4 patch to treat blocking streams in a special way

Motivation for this patch

With this PR, we can use the Symfony Process component. It spawns a new
process via proc_open(), marks its output stream as non-blocking, and
immediately reads the initial output bytes if any are present. Before
this change, Playground would block until the first output is produced,
and, sometimes until the entire process finishes.

This PR is a pre-requisite to #2281

Other changes

This PR implements usleep() (via -Wl,--wrap=usleep). It turns out
that it wasn't actually blocking the execution and a few unit tests
relying on it only passed due to a buggy implementation of blocking
streams.

Follow-up work

• Replace read() globally with something equivalent to wasm_read().
  Right now we only do this RUN /root/replace.sh 's/ret = read/ret = wasm_read/g' /root/php-src/main/streams/plain_wrapper.c
• Consider not overriding php_pollfd_for and removing
  wasm_poll_socket() – wrapping select/poll/read syscalls may be enough
• We could remove more special casing, including socket polling inside
  wasm_poll_socket, the js_popen_to_file function, awaitData et al.

Testing Instructions (or ideally a Blueprint)

Tests are included so just confirm the CI is green.

[adamziel]

It seems like we could just remove our php_pollfd_for override and everything still works. I'll leave that out so to enable finally merging this PR, but that would make a decent follow-up PR.