fixes from coverage testing:    remove redundant check for unused globals    don't complain about unused vars for functions returning None check for unused globals which start with '_'
fixes from coverage testing:
   remove redundant check for unused globals
   don't complain about unused vars for functions returning None
check for unused globals which start with '_'

thomasvs/pychecker,akaihola/PyChecker,akaihola/PyChecker,thomasvs/pychecker