I/O [NT]: Use read-write locking for fd_busy and fd_to_{socket,handle()}.

A negative value in fd_busy now means an exclusive lock (the only kind
that existed before now), 0 no lock, and a positive value shared lock(s).

Fixes issues with concurrent accesses to the same fd from multiple threads.

Potential fix for #10005 where one thread was performing a blocking read
from a socket and another attempted to use it in the write-direction.