Improved handling of the null device
Modified s:last_status value

- fixed type of s:last_status
- set '-1' when exception occurred
Merge pull request #153 from ichizok/null-device

Improve handling of the null device
Merge pull request #154 from ichizok/modify-last_status

Modified s:last_status value
WIP: Improved portability to AIX, HP-UX