dissect: add small "systemd-dissect" tool as wrapper around dissect-image.c

dissect: add small "systemd-dissect" tool as wrapper around dissect-image.c

This adds a small tool that may be used to look into OS images, and mount them
to any place. This is mostly a friendlier version of test-dissect-image.c. I am
not sure this should really become a proper command of systemd, hence for now
do not install it into bindir, but simply libexecdir.

This tool is already pretty useful since you can mount image files with it,
honouring the various partitions correctly. I figure this is going to become
more interesting if the dissctor learns luks and verity support.