util-lib: split out image dissecting code and loopback code from nspawn

This adds two new APIs to systemd:

- loop-util.h is a simple internal API for allocating, setting up and releasing
  loopback block devices.

- dissect-image.h is an internal API for taking apart disk images and figuring
  out what the purpose of each partition is.

Both APIs are basically refactored versions of similar code in nspawn.  This
rework should permit us to reuse this in other places than just nspawn in the
future. Specifically: to implement RootImage= in the service image, similar to
RootDirectory=, but operating on a disk image; to unify the gpt-auto-discovery
generator code with the discovery logic in nspawn; to add new API to machined
for determining the OS version of a disk image (i.e. not just running
containers). This PR does not make any such changes however, it just provides
the new reworked API.

The reworked code is also slightly more powerful than the nspawn original one.
When pointing it to an image or block device with a naked file system (i.e. no
partition table) it will simply make it the root device.

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