Fix for bug #13546 "Build errors with --with-embedded-server

--with-embedded-privilege-control options". One more (hopefully last) build
failure which was introduced during work on WL#2787 "Add view definer/owner
to the view definition..."


libmysqld/lib_sql.cc:
  create_embedded_thd()/check_embedded_connection():
    Several security related THD members (user, host, ip, priv_user, ...) have
    moved to the Security_context class. New THD::security_ctx member points to
    active security context.
sql/sql_acl.cc:
  acl_getroot():
    Updated function description to refelect the fact that THD::user/host/ip/...
    members were moved to separate Security_context class.
sql/sql_parse.cc:
  check_user():
    Updated function description to refelect the fact that THD::user/host/ip/...
    members were moved to separate Security_context class.

sql/sql_parse.cc

@@ -246,7 +246,7 @@ end:
 
   SYNOPSIS
     check_user()
-    thd          thread handle, thd->{host,user,ip} are used
+    thd          thread handle, thd->security_ctx->{host,user,ip} are used
     command      originator of the check: now check_user is called
                  during connect and change user procedures; used for 
                  logging.
@@ -261,8 +261,8 @@ end:
     are 'IN'.
 
   RETURN VALUE
-    0  OK; thd->user, thd->master_access, thd->priv_user, thd->db and
-       thd->db_access are updated; OK is sent to client;
+    0  OK; thd->security_ctx->user/master_access/priv_user/db_access and
+       thd->db are updated; OK is sent to client;
    -1  access denied or handshake error; error is sent to client;
    >0  error, not sent to client
 */