initdb: Use atexit()

initdb: Use atexit()

Replace exit_nicely() calls with standard exit() and register the
cleanup actions using atexit().  The coding pattern used here mirrors
existing use in pg_basebackup.c.

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/