machined: add API for querying the OS release of a machine image

machined: add API for querying the OS release of a machine image

This adds a bus call GetImageOSRelease() to the Manager interface that
retrieves the /etc/os-release file of a machine image. It matches the existing
GetMachineOSRelease() call, however operates on a disk image rather than a
running container.

The backend for this call on .raw images is implemented via the generalized
image dissector, which makes this scheme relatively easy to implement.