gh-109981: Fix support.fd_count() on macOS 14 (#112797)

Use scanning "/dev/fd/" on macOS in support.fd_count(). That's both more efficient than scanning all possible file descriptors, and avoids crashing the interpreter when there are open "guarded" file descriptors.

"Guarded" file descriptors are a macOS feature where file descriptors used by system libraries are marked and cause hard crashes when used by "user" code.

Co-authored-by: Victor Stinner <vstinner@python.org>