Verify usb:// or file:// URIs for problems

[dagwieers]

An annoying configuration error is an insufficient number of slashes in file:// or usb:// URIs. For instance if you configure

BACKUP_URL=usb://dev/disk/by-label/Pack

Rear will bail out with a confusing message that /disk/by-label/Pack could not be found. This can be avoided if we simply check those specific URLs for an empty host part. Which is mandatory for usb:// and file://.