obex: Use user's cache dir as a default root
obex: Use user's cache dir as a default root

It's per-user, so we won't try to overwrite somebody else's
files in /tmp when that happens. It's also (unless we have a
particularly bizarre setup) on the same partition as the destination
folder which means we can atomically move the file to the destination
with a unique filename.

pstglia/external-bluetooth-bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,pkarasev3/bluez,silent-snowman/bluez,mapfau/bluez,mapfau/bluez,silent-snowman/bluez,ComputeCycles/bluez,pkarasev3/bluez,silent-snowman/bluez,ComputeCycles/bluez,pkarasev3/bluez,pkarasev3/bluez,silent-snowman/bluez,ComputeCycles/bluez,mapfau/bluez