gphoto2: Allow toggling libgphoto2 debug output
gphoto2: Allow toggling libgphoto2 debug output

Introduce GVFS_GPHOTO2_DEBUG environment variable to toggle libgphoto2
debug output. Possible values are "all", "data", "debug", or "verbose".

https://bugzilla.gnome.org/show_bug.cgi?id=740660

gicmo/gvfs,gicmo/gvfs,gicmo/gvfs,gicmo/gvfs