Split pg_start_backup() and pg_stop_backup() into two pieces

Split pg_start_backup() and pg_stop_backup() into two pieces

Move the actual functionality into a separate function that's
easier to call internally, and change the SQL-callable function
to be a wrapper calling this.

Also create a pg_abort_backup() function, only callable internally,
that does only the most vital parts of pg_stop_backup(), making it
safe(r) to call from error handlers.