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/transaction.cc

@@ -53,6 +53,9 @@
 #include "src/actions/disruptive/allow.h"
 #include "src/variables/remote_user.h"
 
+#ifdef WIN32
+#include "src/compat/msvc.h"
+#endif
 
 
 using modsecurity::actions::Action;