Fix TLS-PSK identity hint implementation issues.

PSK identity hint can be stored in SSL_CTX and in SSL/SSL_SESSION,
similar to other TLS parameters, with the value in SSL/SSL_SESSION
taking precedence over the one in SSL_CTX. The value in SSL_CTX is
shared (used as the default) between all SSL instances associated
with that SSL_CTX, whereas the value in SSL/SSL_SESSION is confined
to that particular TLS/SSL connection/session.

The existing implementation of TLS-PSK does not correctly distinguish
between PSK identity hint in SSL_CTX and in SSL/SSL_SESSION. This
change fixes these issues:
1. SSL_use_psk_identity_hint does nothing and returns "success" when
   the SSL object does not have an associated SSL_SESSION.
2. On the client, the hint in SSL_CTX (which is shared between
   multiple SSL instances) is overwritten with the hint received from
   server or reset to NULL if no hint was received.
3. On the client, psk_client_callback is invoked with the hint from
   SSL_CTX rather than from current SSL/SSL_SESSION (i.e., the one
   received from the server). Issue #2 above masks this issue.
4. On the server, the hint in SSL/SSL_SESSION is ignored and the hint
   from SSL_CTX is sent to the client.
5. On the server, the hint in SSL/SSL_SESSION is reset to the one in
   SSL_CTX after the ClientKeyExchange message step.

This change fixes the issues by:
* Adding storage for the hint in the SSL object. The idea being that
  the hint in the associated SSL_SESSION takes precedence.
* Reading the hint during the handshake only from the associated
  SSL_SESSION object.
* Initializing the hint in SSL object with the one from the SSL_CTX
  object.
* Initializing the hint in SSL_SESSION object with the one from the
  SSL object.
* Making SSL_use_psk_identity_hint and SSL_get_psk_identity_hint
  set/get the hint to/from SSL_SESSION associated with the provided
  SSL object, or, if no SSL_SESSION is available, set/get the hint
  to/from the provided SSL object.
* Removing code which resets the hint during handshake.

Change-Id: I13f51a5e942269a727c9f26f31155e3d5093903f