Add a `select` function, defined only on platforms where it doesn't
have an `FD_SETSIZE` limitation.

In the `waitpid` tests, ensure that the child process has exited, as
dropping `Command` otherwise leaves the process running. This fixes
test hangs on macos.

And make `select` unsafe due to I/O safety.

Windows has a different `FD_SET` representation. Supporting it might be
possible, though the documentation is ambiguous about whether it supports
arbitrary `FD_SETSIZE` values. But even if so, it would require a more
elaborate abstraction, so just drop it for now.

This uses a trick where we still allow users to allocate a
`FdSetElement` array, but we just allocate a `FD_SET` on Windows
out of it.