Cleanup code for preparsing pg_hba.conf and pg_ident.conf. Store line

Cleanup code for preparsing pg_hba.conf and pg_ident.conf.  Store line
number in the data structure so that we can give at least a minimally
useful idea of where the mistake is when we issue syntax error messages.
Move the ClientAuthentication() call to where it should have been in
the first place, so that postmaster memory releasing can happen in a
reasonable place also.  Update obsolete comments, correct one real bug
(auth_argument was not picked up correctly).