Extend and improve use of EXTRA_REGRESS_OPTS.

This is now used by ecpg tests, and not clobbered by pg_upgrade
tests. This change won't affect anything that doesn't set this
environment variable, but will enable the buildfarm to control
exactly what port regression test installs will be running on,
and thus to detect possible rogue postmasters more easily.

Backpatch to release 9.2 where EXTRA_REGRESS_OPTS was first used.