Add a function to get the authenticated user ID.

 Add a function to get the authenticated user ID.
 
 Previously, this was not exposed outside of miscinit.c.  It is needed
 for the pending pg_background patch, and will also be needed for
 parallelism.  Without it, there's no way for a background worker to
 re-create the exact authentication environment that was present in the
 process that started it, which could lead to security exposures.