Each pclass hold a mapping from type names to nodes of that type
that are currently available for use. This structure was not being
properly maintained; when a mapping was removed, only the entry
for the pnodes *current* type was being put back. When the pnode
goes empty, *all* of its possible types should be marked as available.

The consequence was that for pnodes that could take on multiple types
(which is the common case in Emulab), once a node had been considered
for a certain type (say, 'pc', 'pcvm', or 'delay'), it would *never*
be considered for another type again.

Add a new function that gets called when a pnode goes empty, to reset
*all* of its type mappings in its pclass.

Fixing this bug could have some serious positive consequences.