Add ssh_get_poll_flags()

For integration with an external mainloop, we need to know how to
replicate libssh's internal poll() calls.  We originally through
ssh_get_status() was that API, but it's not really - those flags only
get updated from the *result* of a poll(), where what we really need
is to know how libssh would *start* a poll().

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/socket.h

@@ -52,6 +52,7 @@ void ssh_socket_set_write_wontblock(ssh_socket s);
 void ssh_socket_set_read_wontblock(ssh_socket s);
 void ssh_socket_set_except(ssh_socket s);
 int ssh_socket_get_status(ssh_socket s);
+int ssh_socket_get_poll_flags(ssh_socket s);
 int ssh_socket_buffered_write_bytes(ssh_socket s);
 int ssh_socket_data_available(ssh_socket s);
 int ssh_socket_data_writable(ssh_socket s);