Update to start/stop script's use of 'su' command

Same change now on the develop branch -

The previous syntax of:
su -c 'COMMAND' - USER

Is not valid on Macs and produces an error:
su: illegal option -- c

The correct syntax for the same command on Macs is:
su USER -c 'COMMAND'

This seems to work on Linux as well, I tested the command syntax on Fedora 20.