Just use <wsutil/file_util.h>.

That removes the last bit of Wireshark source that explicitly includes
io.h and most if not all of the code that explicitly includes unistd.h
for read()/write()/lseek()/close().  I'll update the documentation to
recommend including <wsutil/file.h> for all file I/O.

Change-Id: I7c8be50447daba0dc6a78c7331e637468710b2d8
Reviewed-on: https://code.wireshark.org/review/11625
Reviewed-by: Guy Harris <guy@alum.mit.edu>

sync_pipe_write.c

@@ -23,16 +23,10 @@
 
 #include <string.h>
 
-#ifdef HAVE_UNISTD_H
-#include <unistd.h>
-#endif
-
-#ifdef _WIN32
-#include <io.h>
-#endif
-
 #include <glib.h>
 
+#include <wsutil/file_util.h>
+
 #include "sync_pipe.h"
 
 /****************************************************************************************************************/