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/prefs.go

@@ -29,6 +29,7 @@ import (
 	"tailscale.com/types/views"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/syspolicy"
+	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/version"
 )
 
@@ -726,7 +727,7 @@ func (p PrefsView) ControlURLOrDefault() string {
 // If not configured, or if the configured value is a legacy name equivalent to
 // the default, then DefaultControlURL is returned instead.
 func (p *Prefs) ControlURLOrDefault() string {
-	controlURL, err := syspolicy.GetString(syspolicy.ControlURL, p.ControlURL)
+	controlURL, err := syspolicy.GetString(pkey.ControlURL, p.ControlURL)
 	if err != nil {
 		controlURL = p.ControlURL
 	}