Various bug fixes, segfault, USBError access denied etc.

Hi, 
These patches was required to make spread work on my ubuntu 14.04
The workaround for USB error makes spread work out of the box ie: no chmod on /dev/bus/usb and no udev rules (provided camera works in normal gui). 
Though chdkptp was interfered by gvfs--gphoto2-volume-monitor so I had to chmod -x on /usr/lib/gvfs/gvfs-gphoto2-volume-monitor