use mysqld.conf when fetching slave status

we need -u mysql to get super privileges, this is the simpliest way (and
works by default in pld)

service mysql status gives such error
ERROR 1227 (42000) at line 1: Access denied; you need the SUPER,REPLICATION CLIENT privilege for this operation

which works fine from commandline if you have ~/.my.cnf specifing user=mysql:
$ mysql -S /var/lib/mysql/mysqldb/mysql.sock -e show slave status\G

fine tuning could add special user for this purpose only