all: use cmpx.Or where it made sense

I left a few out where writing it explicitly was better
for various reasons.

Updates #8296

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

cmd/derper/depaware.txt

@@ -125,6 +125,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
    W    tailscale.com/util/clientmetric                              from tailscale.com/net/tshttpproxy
         tailscale.com/util/cloudenv                                  from tailscale.com/hostinfo+
    W    tailscale.com/util/cmpver                                    from tailscale.com/net/tshttpproxy
+        tailscale.com/util/cmpx                                      from tailscale.com/cmd/derper+
    L 💣 tailscale.com/util/dirwalk                                   from tailscale.com/metrics
         tailscale.com/util/dnsname                                   from tailscale.com/hostinfo+
         tailscale.com/util/httpm                                     from tailscale.com/client/tailscale

cmd/derper/derper.go

@@ -33,6 +33,7 @@ import (
 	"tailscale.com/net/stun"
 	"tailscale.com/tsweb"
 	"tailscale.com/types/key"
+	"tailscale.com/util/cmpx"
 )
 
 var (
@@ -436,11 +437,7 @@ func defaultMeshPSKFile() string {
 }
 
 func rateLimitedListenAndServeTLS(srv *http.Server) error {
-	addr := srv.Addr
-	if addr == "" {
-		addr = ":https"
-	}
-	ln, err := net.Listen("tcp", addr)
+	ln, err := net.Listen("tcp", cmpx.Or(srv.Addr, ":https"))
 	if err != nil {
 		return err
 	}