Could not add a testcase for this: if the test goes into a MASTER_POS_WAIT, it waits
until this terminates (even doing "connection other_con" to launch "stop slave" is blocked).
- In MASTER_POS_WAIT() don't test if the I/O slave is running, but if the SQL thread
is running.
- Some DBUG info for this bugfix.


sql/slave.cc:
  Fix for bug 651: now a dying SQL slave threads wakes up any waiting MASTER_POS_WAIT().
  In MASTER_POS_WAIT() don't test if the I/O slave is running, but if the SQL thread
  is running.
  Some DBUG info.