util/syspolicy/*: move syspolicy keys to new const leaf "pkey" package

This is step 1 of ~3, breaking up #14720 into reviewable chunks, with
the aim to make syspolicy be a build-time configurable feature.

In this first (very noisy) step, all the syspolicy string key
constants move to a new constant-only (code-free) package. This will
make future steps more reviewable, without this movement noise.

There are no code or behavior changes here.

The future steps of this series can be seen in #14720: removing global
funcs from syspolicy resolution and using an interface that's plumbed
around instead. Then adding build tags.

Updates #12614

Change-Id: If73bf2c28b9c9b1a408fe868b0b6a25b03eeabd1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/desktop/extension.go

@@ -19,6 +19,7 @@ import (
 	"tailscale.com/ipn/ipnext"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/syspolicy"
+	"tailscale.com/util/syspolicy/pkey"
 )
 
 // featureName is the name of the feature implemented by this package.
@@ -135,7 +136,7 @@ func (e *desktopSessionsExt) getBackgroundProfile(profiles ipnext.ProfileStore)
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
-	if alwaysOn, _ := syspolicy.GetBoolean(syspolicy.AlwaysOn, false); !alwaysOn {
+	if alwaysOn, _ := syspolicy.GetBoolean(pkey.AlwaysOn, false); !alwaysOn {
 		// If the Always-On mode is disabled, there's no background profile
 		// as far as the desktop session extension is concerned.
 		return ipn.LoginProfileView{}