[15] Custom WAL resource managers, single user mode, and recovery

Single user mode doesn't process shared_preload_libraries, which is a
problem if custom WAL resource managers are in use and recovery is
required.

I attached a simple patch to process shared_preload_libraries even in
single user mode.

If a user has custom table access methods, those are likely to be
required during single user mode anyway (to VACUUM, for instance), so
we need to be able to load modules during single user mode. I can't
think of a reason specifically why we can't process
shared_preload_libraries. Thoughts?

Regards,
Jeff Davis