Add a SECURITY LABEL command.

This is intended as infrastructure to support integration with label-based
mandatory access control systems such as SE-Linux. Further changes (mostly
hooks) will be needed, but this is a big chunk of it.

KaiGai Kohei and Robert Haas

doc/src/sgml/reference.sgml

@@ -160,6 +160,7 @@
    &rollbackPrepared;
    &rollbackTo;
    &savepoint;
+   &securityLabel;
    &select;
    &selectInto;
    &set;