pgsql: Register atexit hook only once in pg_upgrade.

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>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/93b99d3b6aec67a5eac30c67c511dbb03dd2f72c

Modified Files
--------------
src/bin/pg_upgrade/server.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)