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/audit_log/writer/parallel.cc

@@ -21,7 +21,12 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+#ifndef WIN32
 #include <unistd.h>
+#else
+#include <io.h>
+#include "src/compat/msvc.h"
+#endif
 #include <stdlib.h>
 
 #include <fstream>

src/audit_log/writer/writer.h

@@ -18,8 +18,10 @@
 
 
 #include <stdio.h>
+#ifndef WIN32
 #include <sys/ipc.h>
 #include <sys/shm.h>
+#endif
 #include <sys/types.h>
 
 #include <iostream>