make all mysql command run with --no-defaults; fix #377 regression

in #377 the .my.cnf interference was fixed by always adding `--defaults-file=/dev/null`, later changed to `--no-defaults`.
a9f7b7c76d6e22f38ae922efa86de13f03324c63 refactored it moving `--no-defaults` from a unique location to the single mysql commands but forgot mysqldump and mysqlcheck