Fix check_loopback()

Add missing htonl() so that check_loopback() actually tests for 127.0.0.1
instead of 1.0.0.127 on little-endian machines.
machined: Move image discovery logic into src/shared, so that we can make use of it from nspawn
nspawn: use the same image discovery logic in nspawn as in machined
machined: add "machinectl remove" for removing images
machinectl/machined: implement "rename", "clone", "read-only" verbs for machine images
tmpfiles: add new line type 'v' for creating btrfs subvolumes
tmpfiles.d: upgrade a couple of directories we create at boot to subvolumes

In particular we upgrade /var/lib/container, /var/tmp and /tmp to
subvolumes.
machined: add support for reporting image size via btrfs quota
util: treat -1 as special size in format_bytes()
update TODO
machined: don't look for images on each property get, but cache the image object inbetween