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>

wgengine/router/router_openbsd.go

@@ -14,6 +14,7 @@ import (
 	"go4.org/netipx"
 	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/set"
 )
 
 // For now this router only supports the WireGuard userspace implementation.
@@ -26,7 +27,7 @@ type openbsdRouter struct {
 	tunname string
 	local4  netip.Prefix
 	local6  netip.Prefix
-	routes  map[netip.Prefix]struct{}
+	routes  set.Set[netip.Prefix]
 }
 
 func newUserspaceRouter(logf logger.Logf, tundev tun.Device, netMon *netmon.Monitor) (Router, error) {
@@ -173,9 +174,9 @@ func (r *openbsdRouter) Set(cfg *Config) error {
 		}
 	}
 
-	newRoutes := make(map[netip.Prefix]struct{})
+	newRoutes := set.Set[netip.Prefix]{}
 	for _, route := range cfg.Routes {
-		newRoutes[route] = struct{}{}
+		newRoutes.Add(route)
 	}
 	for route := range r.routes {
 		if _, keep := newRoutes[route]; !keep {