net: change default of the socket used by net.listen_tcp, to dualstac…

…k, even if the OS has a different default. Allow changing the listen backlog too

With this change, example vweb programs, will continue to be available to both
ipv6 and ipv4 connections from the same machine, even after doing (on linux):
`echo 1 | sudo tee /proc/sys/net/ipv6/bindv6only`

Previously, after that, vweb programs responded only to ipv6 connections, but not to ipv4 ones,
i.e. opening http://127.0.0.1:8082/ stopped working, for `v run examples/vweb/vweb_example.v` .

Note: GO web servers have the same behaviour, which is convenient for development/testing,
since it makes the programs more consistent and robust in the face of OS settings changes.

vlib/net/tcp.v

@@ -284,20 +284,27 @@ mut:
 	accept_deadline time.Time
 }
 
-pub fn listen_tcp(family AddrFamily, saddr string) !&TcpListener {
-	s := new_tcp_socket(family) or { return error('${err.msg()}; could not create new socket') }
+[params]
+pub struct ListenOptions {
+pub:
+	dualstack bool = true
+	backlog   int  = 128
+}
+
+pub fn listen_tcp(family AddrFamily, saddr string, options ListenOptions) !&TcpListener {
+	mut s := new_tcp_socket(family) or { return error('${err.msg()}; could not create new socket') }
+	s.set_dualstack(options.dualstack) or {}
 
 	addrs := resolve_addrs(saddr, family, .tcp) or {
 		return error('${err.msg()}; could not resolve address ${saddr}')
 	}
-
 	// TODO(logic to pick here)
 	addr := addrs[0]
 
 	// cast to the correct type
 	alen := addr.len()
 	socket_error_message(C.bind(s.handle, voidptr(&addr), alen), 'binding to ${saddr} failed')!
-	socket_error_message(C.listen(s.handle, 128), 'listening on ${saddr} failed')!
+	socket_error_message(C.listen(s.handle, options.backlog), 'listening on ${saddr} with maximum backlog pending queue of ${options.backlog}, failed')!
 	return &TcpListener{
 		sock: s
 		accept_deadline: no_deadline