Use a new global DEV_NULL instead of hard-coding /dev/null into the system command helper functions.
Use a new global DEV_NULL instead of hard-coding /dev/null into the system
command helper functions.

See #6479 for some motivation.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator