tun-device: Remove the superfluous use of select() before read()
socket-default: Use poll(2) instead of select

It is not only simpler, but also allows the use of arbitrary high fd numbers,
which silently fails with select().
libtls: Use poll(2) instead of select() in tls_socket
watcher: Use poll(2) instead of select
kernel-libipsec: Use poll(2) instead of select
windows: Provide a poll(2) wrapper calling WSAPoll()