Implement server side of PSK extension construction

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2259)

ssl/ssl_err.c

diff --git a/ssl/ssl_err.c b/ssl/ssl_err.c
index 4047d0e..589cfa2 100644
--- a/ssl/ssl_err.c
+++ b/ssl/ssl_err.c
@@ -355,6 +355,7 @@
      "tls_construct_stoc_key_share"},
     {ERR_FUNC(SSL_F_TLS_CONSTRUCT_STOC_NEXT_PROTO_NEG),
      "tls_construct_stoc_next_proto_neg"},
+    {ERR_FUNC(SSL_F_TLS_CONSTRUCT_STOC_PSK), "tls_construct_stoc_psk"},
     {ERR_FUNC(SSL_F_TLS_CONSTRUCT_STOC_RENEGOTIATE),
      "tls_construct_stoc_renegotiate"},
     {ERR_FUNC(SSL_F_TLS_CONSTRUCT_STOC_SERVER_NAME),