Add local TcpSocketPair (#2526)

* Add extern fn socketpair() to posix
* Add extern fn getsockname() for local socketpair loopback in windows
* Add local TcpSocketPair
* Add unit test for TcpSocketPair
* Add implicit wsa startup

---------

Co-authored-by: Christoffer Lerno <christoffer@aegik.com>

test/unit/stdlib/net/tcp_socketpair.c3

@@ -0,0 +1,19 @@
+module tcpsockpairtest @test;
+import std::net;
+
+fn void test_tcp_sock_pair()
+{
+	tcp::TcpSocketPair sockets;
+	sockets.init()!!;
+	defer sockets.destroy()!!;
+
+	String expected = "hello, world!";
+	sockets.send.write(expected)!!;
+
+	char[100] recv_buf;
+	sockets.recv.read(&recv_buf)!!;
+
+	String result = (String) recv_buf[0:expected.len];
+
+	assert(result.trim() == expected);
+}