kes Fix mtx-changer to detect both versions of Ubuntu (Debian)

     mt. This fixes bug #1116.
kes  Add const in python files to reduce compiler warnings.
     This may have to be backed out because of later compiler
     strict checking.
kes  Fix failure of 2drive-concurrent-test. Always read label;
     clear_unload() only after drive is defined in acquire_for_read().