Use os.devnull in place of '/dev/null'.

Although the old code had a fallback that should have worked under
Windows, this approach is cheaper and less error-prone.
call_silently(): Pipe stderr to /dev/null.

It was never used anyway.
Fixed reference to unbound variable.
Merge branch 'devnull'

Fix some uses of /dev/null.
Merge branch 'blitz/typo'

* Fix reference to unbound variable.