shared_preload_libraries is ignored in single user mode

I found out "shared_preload_libraries" setting is ignored when we launch
postgres in single user mode.

In this case, postgres command is launched with "--single" argument,
then the main() directly invokes PostgresMain(); without going through
PostmasterMain() which calls process_shared_preload_libraries().

I think we should put the following code block on somewhere within
PostgresMain() to fix up it.

/*
* If not under postmaster, shared preload libraries are not
* loaded yet, so we try to load them here.
*/
if (!IsUnderPostmaster)
process_shared_preload_libraries();

The reason why I want to use modules in single user mode is to assign
initial security labels on database objects just after initdb.
But, the GUC is ignored, we cannot invokes the routines in the module. :(

Thanks,
--
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>