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).

src/syscall/net.go

@@ -32,3 +32,22 @@ type Conn interface {
 	// SyscallConn returns a raw network connection.
 	SyscallConn() (RawConn, error)
 }
+
+const (
+	AF_INET       = 0x2
+	SOCK_STREAM   = 0x1
+	SOCK_DGRAM    = 0x2
+	SOL_SOCKET    = 0x1
+	SO_KEEPALIVE  = 0x9
+	SOL_TCP       = 0x6
+	TCP_KEEPINTVL = 0x5
+	IPPROTO_TCP   = 0x6
+	IPPROTO_UDP   = 0x11
+	F_SETFL       = 0x4
+
+	// TINYGO: Made up, not a real IP protocol number.  This is used to
+	// create a TLS socket on the device, assuming the device supports mbed
+	// TLS.
+
+	IPPROTO_TLS = 0xFE
+)

src/syscall/syscall_libc_darwin.go

@@ -53,7 +53,6 @@ const (
 	DT_UNKNOWN = 0x0
 	DT_WHT     = 0xe
 	F_GETFL    = 0x3
-	F_SETFL    = 0x4
 	O_NONBLOCK = 0x4
 )
 

src/syscall/syscall_libc_wasi.go

@@ -102,7 +102,6 @@ const (
 
 	// ../../lib/wasi-libc/expected/wasm32-wasi/predefined-macros.txt
 	F_GETFL = 3
-	F_SETFL = 4
 )
 
 // These values are needed as a stub until Go supports WASI as a full target.