gpu-manager.c: clean up get_output()

This should fix LP: 1290831
gpu-manager.c, gpu-manager.py: prevent possible buffer overflows by using fgets
gpu-manager.c: re-enable outputs connected to NVIDIA discrete cards

This only affects Optimus systems.
gpu-manager.c: match only PCI_CLASS_DISPLAY, except for PCI_CLASS_DISPLAY_OTHER
Update changelog