Fix parallel safety markings for pg_start_backup.

Commit 71176854 made pg_start_backup
parallel-restricted rather than parallel-safe, because it now relies
on backend-private state that won't be synchronized with the parallel
worker.  However, it didn't update pg_proc.h.  Separately, Andreas
Karlsson observed that system_views.sql neglected to reiterate the
parallel-safety markings whe redefining various functions, including
this one; so add a PARALLEL RESTRICTED declaration there to match
the new value in pg_proc.h.