pg_upgrade: Handle hash index upgrades more smoothly.

Mark any old hash indexes as invalid so that they don't get used, and
create a script to run REINDEX on all of them.  Without this, we'd
still try to use any upgraded hash indexes, but it would fail.

Amit Kapila, reviewed by me.  Per a suggestion from Tom Lane.

Discussion: http://postgr.es/m/CAA4eK1Jidtagm7Q81q-WoegOVgkotv0OxvHOjFxcvFRP4X=mSw@mail.gmail.com