- User module improvements: added an 'access' column to the users-table to

  keep track of the user's last access. In turn, this allowed me to:

  1. Optimize the "Who's online" block.  On drupal.org, the "Who's online"
     block requires 32 SQL queries.  With this patch, only 2 queries are
     left (eliminated 30 SQL queries), and one of the two remaining queries
     became appr. 20 times faster.

  2. Correct the "Last access" column in the user administration overview
     table.  The presented data was not accurate, which led to the column
     being removed.  You can now sort users by 'last access'.