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>

gokrazy/tsapp/config.json

@@ -12,6 +12,8 @@
     "Packages": [
         "github.com/gokrazy/serial-busybox",
         "github.com/gokrazy/breakglass",
+        "github.com/bradfitz/qemu-guest-kragent",
+        "tailscale.com/cmd/fbstatus",
         "tailscale.com/cmd/tailscale",
         "tailscale.com/cmd/tailscaled"
     ],

gokrazy/tsapp/gokrazydeps.go

@@ -6,6 +6,7 @@
 package gokrazydeps
 
 import (
+	_ "github.com/bradfitz/qemu-guest-kragent"
 	_ "github.com/gokrazy/breakglass"
 	_ "github.com/gokrazy/gokrazy/cmd/dhcp"
 	_ "github.com/gokrazy/gokrazy/cmd/ntp"