WL#944: More information in slow query log

It would be very helpful to have additional information about slow queries,
in slow query logs, which sometimes can't be delivered from explain:

- disk based tmp table
- merge (file) sort

We know this only after query execution.
Other useful might be (they can be delivered from explain)

- full join  
- full table scan

So the event may look like this:

# Time: 030603 16:45:27
# User@Host: pz[pz] @ localhost []
# Query_time: 0  Lock_time: 0  Rows_sent: 100000  Rows_examined: 100000
# Execution flags: full_join,table_scan,merge_sort
use test;
SELECT * FROM A,B order by A.id;