Fix sorting of the uid hash table.

We must not blindy sort the whole table because in interactive mode
this table may be modified by later commands and lookup_uid() gets
confused if the entries have been permuted. So introduce
uid_hash_table_sort_idx, an array of indices that describes the current
sorting of the hash table. The order of the entries in the table itsself
never gets changed any more with this patch.

Also, simplify the uid comparators by introducing a wrapper that does
the casts which were previously contained in each comparator.