replaced hard-coded path with already instantiated var

In main.py, interactions with interface.cfg were hard-coded to /usr/lib/netgui/interface.cfg.  This threw errors in the AUR version, as interface.cfg lives in /var/lib/netgui/interface.cfg.  This location had already been instantiated as intFile.  Therefore all instances of /usr/lib/netgui/interface.cfg were replaced with intFile.