Cygwin: cleanup header including within network-releated files

* Rearrange includes and drop unneccessary ones.

* Don't pull in cygwin/socket.h into sys/un.h just to get
  sa_family_t.  Include sys/types.h and use __sa_family_t instead.

* start including Windows headers using the w32api/ path prefix

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/syslog.cc

@@ -15,6 +15,7 @@ details. */
 #include <stdio.h>
 #include <syslog.h>
 #include <unistd.h>
+#include <sys/socket.h>
 #include <sys/un.h>
 #include "cygerrno.h"
 #include "security.h"