Tune allocation and comparison of nodes

Tune allocation and comparison of nodes
representing temp files.  This improved CPU performance of
'sort -S 1 *.[ch]' by 17% on my host.

(struct tempnode): name member now uses struct hack.
(temphead): Now a pointer, not a structure.  All uses changed.
(create_temp_file): Allocate node using struct hack.
(zaptemp): Free node using struct hack.  Use pointer comparison, not
string comparison.