machined: introduce pseudo-machine ".host" refererring to the host system

machined: introduce pseudo-machine ".host" refererring to the host system

Some of the operations machined/machinectl implement are also very
useful when applied to the host system (such as machinectl login,
machinectl shell or machinectl status), hence introduce a pseudo-machine
by the name of ".host" in machined that refers to the host system, and
may be used top execute operations on the host system with.

This copies the pseudo-image ".host" machined already implements for
image related commands.

(This commit also adds a PK privilege for opening a PTY in a container,
which was previously not accessible for non-root.)