WL#641: Add variable log_rows_examined_limit

Add global and local variable log_rows_examined_limit.
Default value for variable should be LOG_ROWS_EXAMINED_LIMIT = 100

If the number of examined rows is less than this, the query should not get 
logged to the slow log.

This should be done in MySQL 4.0 (Customer request).

This is an easy task as all the number of examined rows is known when the slow 
query log is written.

[Sergei]: consider also a more appropriate name for the variable, e.g.
log_slow_rows_examined_limit - to show that it is slow log related

The variable min_examined_row_limit added in 5.1 solves this problem