polarssl: fix possible handshake timeout issue in multi.

Because of the socket is unblocking, PolarSSL does need call to getsock to
get the action to perform in multi environment.

In some cases, it might happen we have not received yet all data to perform
the handshake. ssh_handshake returns POLARSSL_ERR_NET_WANT_READ, the state
is updated but because of the getsock has not the proper #define macro to,
the library never prevents to select socket for input thus the socket will
never be awaken when last data is available. Thus it leads to timeout.