Free allocated memory on exit

This should reduce the number of possible memory leaks reported by
valgrind. The remaining reported leaks are sched tqe allocation, async
DNS instance allocation, cmdmon response/timestamp cell allocation, and
clientlog subnet allocation.

hash.h

@@ -38,4 +38,6 @@ extern unsigned int HSH_Hash(int id,
     const unsigned char *in2, unsigned int in2_len,
     unsigned char *out, unsigned int out_len);
 
+extern void HSH_Finalise(void);
+
 #endif