Add -fsys.addr flag for Windows

This allows user to listen to a specified port instead of a random one.

fsys_windows.go

@@ -1,22 +1,26 @@
 package main
 
 import (
+	"flag"
 	"fmt"
 	"net"
 
 	"9fans.net/go/plan9/client"
 	"github.com/fhs/mux9p"
 )
 
-var fsysAddr net.Addr
+var (
+	fsysAddrFlag = flag.String("fsys.addr", "localhost:0", "9P file system listen address")
+	fsysAddr     net.Addr
+)
 
 func newPipe() (net.Conn, net.Conn, error) {
 	c1, c2 := net.Pipe()
 	return c1, c2, nil
 }
 
 func post9pservice(conn net.Conn, name string, mtpt string) error {
-	l, err := net.Listen("tcp", "localhost:0")
+	l, err := net.Listen("tcp", *fsysAddrFlag)
 	if err != nil {
 		return fmt.Errorf("listen failed: %v", err)
 	}