dissect-image: introduce new get_common_dissect_directory() helper

dissect-image: introduce new get_common_dissect_directory() helper

So far, if some component mounts a DDI in some local mount namespace we
created a temporary mountpoint in /tmp/ for that. Let's instead use the
same directory inode in /run/ instead. This is safe, since if everything
runs in a local mount namespace (with propagation on /run/ off) then
they shouldn't fight for the inode. And it relieves us from having to
clean up the directory after use. Morever, it allows us to run without
/tmp/ mounted.

This only moves dissect-image.c and the dissec tool over. More stuff is
moved over later.