Volatile-qualify the ProcArray PGPROC pointer in a bunch of routines

Volatile-qualify the ProcArray PGPROC pointer in a bunch of routines
that examine fields that could change under them.  This is just to make
really sure that when we are fetching a value 'only once', that's what
actually happens.  Possibly this is a bug that should be back-patched,
but in the absence of solid evidence that it's needed, I won't bother.