Improve save API by always updating olen

This allows callers to discover what an appropriate size is. Otherwise they'd
have to either try repeatedly, or allocate an overly large buffer (or some
combination of those).

Adapt documentation an example usage in ssl_client2.

include/mbedtls/ssl.h

@@ -2391,10 +2391,16 @@ int mbedtls_ssl_session_load( mbedtls_ssl_session *session,
  *
  * \param session  The session structure to be saved.
  * \param buf      The buffer to write the serialized data to. It must be a
- *                 writeable buffer of at least \p len bytes.
+ *                 writeable buffer of at least \p len bytes, or may be \c
+ *                 NULL if \p len is \c 0.
  * \param buf_len  The number of bytes available for writing in \p buf.
- * \param olen     The size of the written data in bytes. It must point to a
- *                 valid \c size_t.
+ * \param olen     The size in bytes of the data that has been or would have
+ *                 been written. It must point to a valid \c size_t.
+ *
+ * \note           \p olen is updated to the correct value regardless of
+ *                 whether \p buf_len was large enough. This makes it possible
+ *                 to determine the necessary size by calling this function
+ *                 with \p buf set to \c NULL and \p buf_len to \c 0.
  *
  * \return         \c 0 if successful.
  * \return         #MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL if \p buf is too small.