transaction: Add HIF_TRANSACTION_FLAG_OSTREE_MODE

[cgwalters]

The goal of this patch is to help support rpm-ostree package layering.
The way this works is that we unpack layered packages on top of a new
hardlinked tree. However, if any packages had scriptlets that
mutate a file in place (as opposed to make-new-then-rename), that
would corrupt the object store and potentially break rollback.

It turns out that most scripts in Fedora are safe, but ultimately
the way I think this should work is that only scripts in e.g.
/usr/share/rpm/scriptlets
which are known verified safe can be run.

Anyways, if set, HIF_TRANSACTION_FLAG_OSTREE_MODE does basically two things:

• Verifies before installing a package (while we have it open for GPG checks)
  that a package has no scripts
• Disables all scripts in the RPM transaction

We can't just do the latter as that would allow installing packages
that didn't work (e.g. shared libraries without running ldconfig).

[cgwalters]

(Updated to fix regression with GPG checking)

[cgwalters]

The intertia around the whole ecosystem is so powerful here that I've put this approach on hold, and am now planning to do coreos/rpm-ostree#107 (comment)

So let's close this for now without merging, if we decide to do it later, the code will still be here.