Change ptopgen to look at the eventstate of a node; a node is not considered
free unless it is ISUP or PXEWAIT.

Add TBAvailablePCs() to libdb, removing the corresponding code from
assign_wrapper. This routine does the equiv of ptopgen, returning the
number of PCs that are available for use (looking at eventstate).

Change TBFreePCs in the web interface accordingly.

The above changes correspond to an upcoming change in stated.