safesocket, ipn/ipnserver: look up peer creds on Darwin

And open up socket permissions like Linux, now that we know who
connections are from.

This uses the new inet.af/peercred that supports Linux and Darwin at
the moment.

Fixes #1347
Fixes #1348

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

ipn/ipnserver/server.go

@@ -26,6 +26,7 @@ import (
 
 	"go4.org/mem"
 	"inet.af/netaddr"
+	"inet.af/peercred"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnlocal"
@@ -309,6 +310,19 @@ func (s *server) serveConn(ctx context.Context, c net.Conn, logf logger.Logf) {
 	}
 }
 
+func isReadonlyConn(c net.Conn, logf logger.Logf) bool {
+	creds, err := peercred.Get(c)
+	if err != nil {
+		return true // conservatively
+	}
+	uid, ok := creds.UserID()
+	if !ok {
+		return true // conservatively
+	}
+	logf("connection from userid %v", uid)
+	return uid != "0"
+}
+
 // inUseOtherUserError is the error type for when the server is in use
 // by a different local user.
 type inUseOtherUserError struct{ error }