net: implement ResolveIPAddr with stub for lookupProtocol, take #2

This is a rework of PR#24 based on review comments.  The rework mostly
involved moving changes to the proper files, aligning with upstream Go
src/net file layout.

lookup_unix.go

@@ -0,0 +1,20 @@
+// TINYGO: The following is copied and modified from Go 1.22.0 official implementation.
+
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || js || wasip1
+
+package net
+
+import (
+	"context"
+	"errors"
+)
+
+// lookupProtocol looks up IP protocol name in /etc/protocols and
+// returns correspondent protocol number.
+func lookupProtocol(_ context.Context, name string) (int, error) {
+	return 0, errors.New("net:lookupProtocol not implemented")
+}