loader: Add support for booting from a ZFS snapshot

When booting from a snapshot we need to follow a different code path
to turn the objset ID into the name, and for forward lookups we need
to walk the parent's snapnames_zap.

With this, it is possible to set the pools BOOTFS property to a
snapshot and boot with a read-only filesystem of that snapshot.

Reviewed by: tsoome, rew, imp
Sponsored By: Beckhoff Automation GmbH & Co. KG
Sponsored By: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38600