(Merge of code from 4.0)


Docs/manual.texi:
  Changelog
include/my_base.h:
  Fixed bug with GROUP BY on NULL fields.
include/mysql_com.h:
  Fixed bug with GROUP BY on NULL fields.
myisam/mi_search.c:
  Fixed bug with GROUP BY on NULL fields.
myisam/mi_write.c:
  Fixed bug with GROUP BY on NULL fields.
mysql-test/r/group_by.result:
  Fixed bug with GROUP BY on NULL fields.
mysql-test/t/group_by.test:
  Fixed bug with GROUP BY on NULL fields.
sql/sql_base.cc:
  Removed some not-needed comments in the trace file
sql/sql_select.cc:
  Fixed bug with GROUP BY on NULL fields.