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/parser/seclang-scanner.cc

@@ -4952,7 +4952,11 @@ static std::stack<int> YY_PREVIOUS_STATE;
  * The user has a chance to override it with an option.
  */
 /* %if-c-only */
+#ifndef WIN32
 #include <unistd.h>
+#else
+#include <io.h>
+#endif
 /* %endif */
 /* %if-c++-only */
 /* %endif */