Make truncated hmac a runtime option server-side

Reading the documentation of ssl_set_truncated_hmac() may give the impression
I changed the default for clients but I didn't, the old documentation was
wrong.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 6d8626c..534525a 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -629,7 +629,8 @@
 
     ((void) buf);
 
-    ssl->session_negotiate->trunc_hmac = SSL_TRUNC_HMAC_ENABLED;
+    if( ssl->trunc_hmac == SSL_TRUNC_HMAC_ENABLED )
+        ssl->session_negotiate->trunc_hmac = SSL_TRUNC_HMAC_ENABLED;
 
     return( 0 );
 }