pg_upgrade: Fix for changed pg_ctl default stop mode

In 9.5, the default pg_ctl stop mode was changed from "smart" to "fast".
pg_upgrade still thought the default mode was "smart" and only specified
the mode when "fast" was asked for.  This results in using "fast" all
the time.  It's not clear what the effect in practice is, but fix it
nonetheless to restore the previous behavior.