Add network device driver model, netdev

This PR adds a network device driver model called netdev. There will be a companion PR for TinyGo drivers to update the netdev drivers and network examples. This PR covers the core "net" package.

An RFC for the work is here: #tinygo-org/drivers#487. Some things have changed from the RFC, but nothing major.

The "net" package is a partial port of Go's "net" package, version 1.19.3. The src/net/README file has details on what is modified from Go's "net" package.

Most "net" features are working as they would in normal Go. TCP/UDP/TLS protocol support is there. As well as HTTP client and server support. Standard Go network packages such as golang.org/x/net/websockets and Paho MQTT client work as-is. Other packages are likely to work as-is.

Testing results are here (https://docs.google.com/spreadsheets/d/e/2PACX-1vT0cCjBvwXf9HJf6aJV2Sw198F2ief02gmbMV0sQocKT4y4RpfKv3dh6Jyew8lQW64FouZ8GwA2yjxI/pubhtml?gid=1013173032&single=true).

iprawsock.go

@@ -1,4 +1,4 @@
-// The following is copied from Go 1.16 official implementation.
+// TINYGO: The following is copied and modified from Go 1.19.3 official implementation.
 
 // Copyright 2010 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
@@ -6,6 +6,24 @@
 
 package net
 
+// BUG(mikio): On every POSIX platform, reads from the "ip4" network
+// using the ReadFrom or ReadFromIP method might not return a complete
+// IPv4 packet, including its header, even if there is space
+// available. This can occur even in cases where Read or ReadMsgIP
+// could return a complete packet. For this reason, it is recommended
+// that you do not use these methods if it is important to receive a
+// full packet.
+//
+// The Go 1 compatibility guidelines make it impossible for us to
+// change the behavior of these methods; use Read or ReadMsgIP
+// instead.
+
+// BUG(mikio): On JS and Plan 9, methods and functions related
+// to IPConn are not implemented.
+
+// BUG(mikio): On Windows, the File method of IPConn is not
+// implemented.
+
 // IPAddr represents the address of an IP end point.
 type IPAddr struct {
 	IP   IP