Add comments.


sql/ha_ndbcluster_binlog.cc:
  query_error -> slave_error
sql/handler.cc:
  query_error -> slave_error
sql/log.cc:
  query_error -> slave_error
sql/log_event.cc:
  query_error -> slave_error
sql/log_event_old.cc:
  query_error -> slave_error
sql/mysqld.cc:
  query_error -> slave_error
sql/protocol.cc:
  query_error -> slave_error
sql/slave.cc:
  query_error -> slave_error
sql/sp_head.cc:
  query_error -> slave_error
sql/sql_class.cc:
  query_error -> slave_error
sql/sql_class.h:
  Rename: query_error -> is_slave_error, to avoid confusion.
  Add commenta.
sql/sql_connect.cc:
  Rename: query_error -> is_slave_error, to avoid confusion.
  Originally it was the same code to handle init-connect and init-slave 
  mysqld options. Then init-connect implementation forked off,
  but the one who copy-pasted the code didn't change it to not
  use a replication-specific variable.