Updated included headers to support compilation on Windows (using Visual C++)

- most of posix related functions and constants in unistd.h can be
  found in io.h in Visual C++
- introduced src/compat/msvc.h to adjust for compiler differences (and
  avoid updating code with #ifdef blocks for Windows support)
- removed some included headers that are not needed (both on Unix and
  Windows builds)

src/utils/msc_tree.cc

@@ -15,9 +15,15 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifndef WIN32
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
+#else
+#include "src/compat/msvc.h"
+#include <winsock2.h>
+#include <ws2tcpip.h>
+#endif
 
 #include "src/utils/msc_tree.h"