vmspawn: use our own ptyfwd code for the console of a VM

vmspawn: use our own ptyfwd code for the console of a VM

Let's make systemd-nspawn use our own ptyfwd logic to handle the TTY by
default.

This adds a new setting --console=, inspired by nspawn's setting of the
same name. If --console=interactive= is used, then we'll do the TTY
dance on our own via ptyfwd, and thus get tinting, our usual hotkey
handling and similar.

Since qemu's own console is useful too, let's keep it around via
--console=native.

FInally, replace the --qemu-gui switch by --console=gui.