Develop subcommand to shell into images

By way of simulated help text, the feature should behave as follows:

Usage:
 nest shell [options] NAME

Options:
  -b, [--boot-env]     # NAME is a boot environment to automatically mount and unmount
  -m, [--mnt]          # NAME is a root mounted under /mnt
  -h, [--host]         # NAME is a host image at /nest/hosts
  -i, [--image]        # NAME is a Nest container image (e.g. 'stage1' or 'tools/pdk')
  -c, [--command=CMD]  # Pass CMD to run in the shell
  -H, [--no-home]      # Do not map homes from the host into the shell
  -O, [--no-overlay]   # Write changes to underlying image (no-op for containers)

Description:
  Launch a tmux session or a command in a Nest image. When a target type is not specified,
  try them in the following order:

  * boot environment (-b)
  * mountpoint under /mnt (-m)
  * host image under /nest/hosts (-h)
  * Nest container image (-i)

Run systemd-nspawn(1) with --volatile=overlay unless --no-overlay is specified.