docs,test: --bind-volume / bind-volume / unbind-volume

docs,test: --bind-volume / bind-volume / unbind-volume

  - Document the new --bind-volume= option in systemd-vmspawn(1) and
    the new bind-volume / unbind-volume verbs in machinectl(1).

  - Add an integration test
    (TEST-87-AUX-UTILS-VM.bind-volume.sh) covering boot-time attach
    via --bind-volume, runtime attach via 'machinectl bind-volume',
    runtime detach via 'machinectl unbind-volume', the StorageImmutable
    rejection of attempts to detach boot-time volumes, and the
    NoSuchStorage rejection of detach on unknown names.

  - Strike "hook-up in systemd-vmspawn" from TODO.md; the nspawn and
    service-manager hookups remain.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>