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>

flakehashes.json

@@ -4,7 +4,7 @@
     "sri": "sha256-cY5yryX+p/xtoTv+WZEKFagiIl0OREHnJY1Bk5VpVVc="
   },
   "vendor": {
-    "goModSum": "sha256-evStNa6VrP7dYOuUIIj8nFPCFVUJ/GzcAefl2C4h8QA=",
-    "sri": "sha256-UrvJ5fM+Oqgu2pZwhg5AnUcgi8wPwZ8qDwWpXNmKaPk="
+    "goModSum": "sha256-gFAlZFTtqRlHzg0hHneMjNpkJ/CLuHEmeEeKT6fHRm0=",
+    "sri": "sha256-sWU4abv9Oz7P21ivL5zgdYNGiJSXamnQR0VmRGKoIrI="
   }
 }