Although we perform path compression to reduce the height of the trie and
the number of interior nodes, we always create a level zero interior node at
the root of every non-empty trie, even when that node is not strictly
necessary, i.e., it has only one child.  This change is the first step in
eliminating those unnecessary level zero interior nodes.  Specifically, it
updates all of the lookup functions so that they do not require a level zero
interior node at the root.

Reviewed by:	attilio, jeff (an earlier version)
Sponsored by:	EMC / Isilon Storage Division
Cosmetic change: make a comment reference Sandy Bridge *Xeon*

Reviewed by:	sbruno
MFC after:	1 week