New 'exec' command

Add new support for running commands or a shell in Nest images, whether they are container images (Podman) or root disk images (systemd-nspawn). Map in useful data by default, such as Portage, ssh-agent, X11, /home, and /nest. Provide options for disabling any of those data maps.

Also, modify the installer to use this new runtime system.

Closes #8 (closed) and #13 (closed).