cmd/tailscaled: default to userspace-networking mode on gokrazy, set paths

One of the current few steps to run Tailscale on gokrazy is to
specify the --tun=userspace-networking flag:

    https://gokrazy.org/userguide/install/tailscale/

Instead, make it the default for now. Later we can change the
default to kernel mode if available and fall back to userspace
mode like Synology, once #391 is done.

Likewise, set default paths for Gokrazy, as its filesystem hierarchy
is not the Linux standard one. Instead, use the conventional paths as
documented at https://gokrazy.org/userguide/install/tailscale/.

Updates #1866

RELNOTE=default to userspace-networking mode on gokrazy

Change-Id: I3766159a294738597b4b30629d2860312dbb7609
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

paths/paths_unix.go

@@ -14,6 +14,7 @@ import (
 	"runtime"
 
 	"golang.org/x/sys/unix"
+	"tailscale.com/version/distro"
 )
 
 func init() {
@@ -34,6 +35,9 @@ func statePath() string {
 }
 
 func stateFileUnix() string {
+	if distro.Get() == distro.Gokrazy {
+		return "/perm/tailscaled/tailscaled.state"
+	}
 	path := statePath()
 	if path == "" {
 		return ""