Fix a design bug concerning struct user_info.

Fix a design bug concerning struct user_info.

It was a bad idea to include the accounting data (#files, #dirs,
information about the user only.

So move the accounting data to user_summary_info. This allows
to get rid of the ugly uid_hash_table_sort_idx construct and of
sort_hash_table(). These were only needed because of the broken
design.  We now never sort the hash table but allocate an array of
user_summary_info structures on each query and sort that array instead.

This patch not only simplifies code but also fixes a real bug noted by
Sebastian Stark: If the user_summary was requested more than once in
interactive mode, the old code computed incorrect values because the
above mentioned accounting fields were only initialized once. The new
code gets this right automatically because a fresh array is created
on each query.