Label switcher function

The attached patch allows the security label provider to switch
security label of the client during execution of certain functions.
I named it as "label switcher function"; also called as "trusted-
procedure" in SELinux community.

This feature is quite similar idea toward security definer function,
or set-uid program on operating system. It allows label providers
to switch its internal state that holds security label of the
client, then restore it.
If and when a label provider said the function being invoked is
a label-switcher, fmgr_security_definer() traps this invocation
and set some states just before actual invocations.

We added three new hooks for security label provider.
The get_client_label and set_client_label allows the PG core to
save and restore security label of the client; which is mostly
just an internal state of plugin module.
And, the get_switched_label shall return NULL or a valid label
if the supplied function is a label switcher. It also informs
the PG core whether the function is switcher or not.

Thanks,
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>