Register atexit hook only once in pg_upgrade.

start_postmaster() registered stop_postmaster_atexit as an atexit(3)
callback each time through, although the obvious intention was to do
so only once per program run.  The extra registrations were harmless,
so long as we didn't exceed ATEXIT_MAX, but still it's a bug.

Artur Zakirov, with bikeshedding by Kyotaro Horiguchi and me

Discussion: <d279e817-02b5-caa6-215f-cfb05dce109a@postgrespro.ru>