polling on mix of libc-posix socket/file descriptors with non-libc WASI pollable handles

In context of dotnet sockets I would like to be able to wasi:poll_poll on mix of handles

• which were created by consuming other WASI APIs, like wasi:http
• and file handles obtained by using libc sockets APIs

The motivation to do it on single "system" call is to

• be able to make progress on events from HTTP while socket is not ready and vice versa
  • if we called each select/poll or poll_poll separately, we would get blocked only on part of pollables
• to be able to block (stop spinning) if there are no I/O events (and no CPU bound jobs)

At the moment, we can hack it via using libc internal descriptor_table_get_ref and it's data structures to obtain underlying pollables.
@dicej made similar hack for mio
But it's fragile and bad practice.

I can see few possible solutions

• expose libc function accepting both lists of resources
  • void wasi_poll_plus_fd(poll_list_borrow_pollable_t *in, list_pollfd_t *in, wasip2_list_u32_t *ret)
  • this is with wit-bindgen generated C structures on APIs and with list of pollfd struct
• we can try simpler C types, something like
  • void wasi_poll_plus_fd(int *pollables, int pollables_count, struct pollfd *fds, int fds_count, int **ret, int *ret_count)
• or we can expose just the mapping from fds to list of pollables
  • void socket_fds_to_wasi_pollables(struct pollfd *fds, int fds_count, int **pollables, int pollables_count)
  • this would have to be called before each call to wasi:poll_poll
• create libc file descriptor by registering external pollable
  • this would have to be un-registered later
  • calling socket oriented poll() API on any pollable (clock) feels confusing to me

Relevant code, possibly to be refactored and reused for above.
https://github.com/WebAssembly/wasi-libc/blob/7d4d3b83fc66c79b3faa5989e67ed2d1042dacaf/libc-bottom-half/sources/poll-wasip2.c#L27-L151

cc @badeend

Start by reading libc-bottom-half/sources/poll-wasip2.c, especially lines 27-151, and review the linked dotnet sockets and mio context. Compare the proposed approaches for combining libc file descriptors with WASI pollables; done means an agreed, supported API design and implementation path rather than reliance on libc internals.