cmd/tailscale: add 'configure pve-appliance' to make Proxmox VM of appliance

This is a variant of "tailscale configure flash-appliance" but for running
on Proxmox PVE hosts to make a Proxmox VM running the experimental
Tailscale Appliance.

This also makes the "Esc" key make the fbstatus GUI open up a terminal,
instead of Control-Alt-F2 which is hard to type over NoVNC.

And make gafpush unidirectional, to not require a local port be opened locally,
which I hit while working on this.

And make fbstatus included in all appliance variants, but bail out early
and stop respawing if the machine has no framebuffer (e.g. AWS VMs).

Updates #1866

Change-Id: I18ec2a16e4d5ff5574e16fe55c0e8d06cf4fab7f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

go.sum

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