Problem:

The "Slave I/O thread couldn't register on master" error sporadically
occurred in replication tests because the slave I/O thread got
killed by STOP SLAVE before or while registering on master.

Fixed by checking the state of the I/O thread, and issueing
the error only if it was not explicitely killed by a user.


sql/slave.cc:
  When the slave I/O thread fails to register on master, issue an error
  message only if it is not explicitely killed by a user with STOP SLAVE.