all: use set.Set consistently instead of map[T]struct{}

I didn't clean up the more idiomatic map[T]bool with true values, at
least yet.  I just converted the relatively awkward struct{}-valued
maps.

Updates #cleanup

Change-Id: I758abebd2bb1f64bc7a9d0f25c32298f4679c14f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/nrpt_windows.go

@@ -13,6 +13,7 @@ import (
 	"golang.org/x/sys/windows/registry"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/dnsname"
+	"tailscale.com/util/set"
 	"tailscale.com/util/winutil"
 )
 
@@ -158,14 +159,14 @@ func (db *nrptRuleDatabase) detectWriteAsGP() {
 	}
 
 	// Add *all* rules from the GP subkey into a set.
-	gpSubkeyMap := make(map[string]struct{}, len(gpSubkeyNames))
+	gpSubkeyMap := make(set.Set[string], len(gpSubkeyNames))
 	for _, gpSubkey := range gpSubkeyNames {
-		gpSubkeyMap[strings.ToUpper(gpSubkey)] = struct{}{}
+		gpSubkeyMap.Add(strings.ToUpper(gpSubkey))
 	}
 
 	// Remove *our* rules from the set.
 	for _, ourRuleID := range db.ruleIDs {
-		delete(gpSubkeyMap, strings.ToUpper(ourRuleID))
+		gpSubkeyMap.Delete(strings.ToUpper(ourRuleID))
 	}
 
 	// Any leftover rules do not belong to us. When group policy is being used