Cleanup of slave code:

- Added testing if connection is killed to shortcut reading of connection data
  This will allow us later in 10.2 to do a cleaner shutdown of slaves (less errors in the log)
- Add new status variables: Slaves_connected, Slaves_running and Slave_connections.
- Use MYSQL_SLAVE_NOT_RUN instead of 0 with slave_running.
- Don't print obvious extra warnings to the error log when slave is shut down normally.

sql/sql_class.h

@@ -656,6 +656,7 @@ typedef struct system_status_var
 {
   ulong com_other;
   ulong com_stat[(uint) SQLCOM_END];
+  ulong com_register_slave;
   ulong created_tmp_disk_tables_;
   ulong created_tmp_tables_;
   ulong ha_commit_count;