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).

README.md

@@ -0,0 +1,107 @@
+This is a port of Go's "net" package.  The port offers a subset of Go's "net"
+package.  The subset maintains Go 1 compatiblity guarantee.
+
+The "net" package is modified to use netdev, TinyGo's network device driver interface.
+Netdev replaces the OS syscall interface for I/O access to the networking
+device.
+
+#### Table of Contents
+
+- ["net" Package](#net-package)
+- [Netdev and Netlink](#netdev-and-netlink)
+- [Using "net" and "net/http" Packages](#using-net-and-nethttp-packages)
+
+## "net" Package
+
+The "net" package is ported from Go 1.19.3.  The tree listings below shows the
+files copied.  If the file is marked with an '\*', it is copied _and_ modified
+to work with netdev.  If the file is marked with an '+', the file is new.  If
+there is no mark, it is a straight copy.
+
+```
+src/net
+├── dial.go			*
+├── http
+│   ├── client.go		*
+│   ├── clone.go
+│   ├── cookie.go
+│   ├── fs.go
+│   ├── header.go		*
+│   ├── http.go
+│   ├── internal
+│   │   ├── ascii
+│   │   │   ├── print.go
+│   │   │   └── print_test.go
+│   │   ├── chunked.go
+│   │   └── chunked_test.go
+│   ├── jar.go
+│   ├── method.go
+│   ├── request.go		*
+│   ├── response.go		*
+│   ├── server.go		*
+│   ├── sniff.go
+│   ├── status.go
+│   ├── transfer.go		*
+│   └── transport.go		*
+├── ip.go
+├── iprawsock.go		*
+├── ipsock.go			*
+├── mac.go
+├── mac_test.go
+├── netdev.go			+
+├── net.go			*
+├── parse.go
+├── pipe.go
+├── README.md
+├── tcpsock.go			*
+├── tlssock.go			+
+└── udpsock.go			*
+
+src/crypto/tls/
+├── common.go			*
+└── tls.go			*
+```
+
+The modifications to "net" are to basically wrap TCPConn, UDPConn, and TLSConn
+around netdev socket calls.  In Go, these net.Conns call out to OS syscalls for
+the socket operations.  In TinyGo, the OS syscalls aren't available, so netdev
+socket calls are substituted.
+
+The modifications to "net/http" are on the client and the server side.  On the
+client side, the TinyGo code changes remove the back-end round-tripper code and
+replaces it with direct calls to TCPConns/TLSConns.  All of Go's http
+request/response handling code is intact and operational in TinyGo.  Same holds
+true for the server side.  The server side supports the normal server features
+like ServeMux and Hijacker (for websockets).
+
+### Maintaining "net"
+
+As Go progresses, changes to the "net" package need to be periodically
+back-ported to TinyGo's "net" package.  This is to pick up any upstream bug
+fixes or security fixes.
+
+Changes "net" package files are marked with // TINYGO comments.
+
+The files that are marked modified * may contain only a subset of the original
+file.  Basically only the parts necessary to compile and run the example/net
+examples are copied (and maybe modified).
+
+## Netdev and Netlink
+
+Netdev is TinyGo's network device driver model.  Network drivers implement the
+netdever interface, providing a common network I/O interface to TinyGo's "net"
+package.  The interface is modeled after the BSD socket interface.  net.Conn
+implementations (TCPConn, UDPConn, and TLSConn) use the netdev interface for
+device I/O access.
+
+Network drivers also (optionally) implement the Netlinker interface.  This
+interface is not used by TinyGo's "net" package, but rather provides the TinyGo
+application direct access to the network device for common settings and control
+that fall outside of netdev's socket interface.
+
+See the README-net.md in drivers repo for more details on netdev and netlink.
+
+## Using "net" and "net/http" Packages
+
+See README-net.md in drivers repo to more details on using "net" and "net/http"
+packages in a TinyGo application.