Refactor FirstHopId into type-differentiated form

The FirstHopId type now records an enum that stores whether the hop
is a guard or a fallback.  This change addresses concerns about
remembering to check the type or source of an Id before passing it
down to the FallbackState or GuardSet.

Making this change required an API change, so that dirmgr can
report success/failure status without actually knowing whether it's
using a fallback or a guard.