additional fixes for BDB and correct assignment of both signed
and unsigned 64-bit data to unsigned system variables


mysql-test/r/ps_2myisam.result:
  account for UNSIGNED_FLAG
mysql-test/r/ps_3innodb.result:
  account for UNSIGNED_FLAG
mysql-test/r/ps_4heap.result:
  account for UNSIGNED_FLAG
mysql-test/r/ps_5merge.result:
  account for UNSIGNED_FLAG
mysql-test/r/ps_6bdb.result:
  account for UNSIGNED_FLAG
mysql-test/r/ps_7ndb.result:
  account for UNSIGNED_FLAG
mysys/my_getopt.c:
  We have correct signed/unsigned information now, so we no longer
  need to err on the side of caution.
sql/item_func.cc:
  Copy unsigned info over from entry so the item's data
  correctly describe it.
sql/mysqld.cc:
  BDB log buffer size: default can't be less than minimum
sql/set_var.cc:
  Handle signedness of in-values correctly when assigning to
  unsigned types, all the way up to 64-bit. Use handler from
  all three unsigned sysvar types.
sql/set_var.h:
  thd_ulonglong: Override default check with one for unsigned types