kgio 2.3.0 - MSG_PEEK and poll(2) support

recv() with MSG_PEEK for sockets is added with the try*
interface.  SocketMethods#kgio_trypeek and
SocketMethods#kgio_peek or Kgio.trypeek for non-Kgio-enabled
sockets.

For Ruby 1.9 only: poll(2) is exposed via the Kgio.poll
singleton method and should provide an alternative for IO.select
users.

Both of these new features should work well on modern Unix-like
operating systems.