pg_basebackup: Use atexit()

Instead of using our custom disconnect_and_exit(), just register the
desired cleanup using atexit() and use the standard exit() to leave
the program.

Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/ec4135ba-84e9-28bf-b584-0e78d47448d5@2ndquadrant.com/

Author: petere