x509roots: support constrained roots

Adds support for roots with the distrust-after bit set. The constraint
function construction is a little funky, but I couldn't think of an
obvious better way to do it.

Fixes golang/go#70777
Fixes golang/go#70623

Change-Id: I780f866416b626360eaee9368185768da7bc75ef
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/652996
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Daniel McCarney <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: rolandshoemaker