[threadpool-io] Throw if backend won't be able to register a FD (mono/mono#16396)

[threadpool-io] Throw if backend won't be able to register a FD (mono/mono#16396)

The poll+select i/o selector backend can't handle file descriptor ids greater
than FD_SETSIZE.  This can happen if too many files are open and we want to
wait on it.

Previously, mono would fail in the i/o selector thread by which point it was
too late to do anything.

With this change we will fail eagerly on the thread that calls IOSelector.Add
by throwing a NotSupportedException.

Addresses https://github.com/mono/mono/issues/15931

Commit migrated from https://github.com/mono/mono/commit/78edafd6e2d5ce886e2fd2ddc3cf8bbbffeeb860