cleanup of register_slave_on_master()


mysql-test/r/rpl000002.result:
  new output of show slave hosts
sql/mysqld.cc:
  user and password are shown in show slave hosts only if show-slave-auth-info option is on
sql/slave.cc:
  cleanup of register_slave_on_master()
sql/sql_repl.cc:
  do not show user/pass for slave unless running with show-slave-auth-info
sql/sql_repl.h:
  show-slave-auth-info