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/compat/msvc.h

@@ -0,0 +1,22 @@
+#ifndef __COMPAT_MSVC
+#define __COMPAT_MSVC
+
+#include <time.h>
+
+#if !defined(S_ISREG) && defined(S_IFMT) && defined(S_IFREG)
+#define S_ISREG(m) (((m) & S_IFMT) == S_IFREG)
+#endif
+
+#define strcasecmp   _stricmp
+#define strncasecmp  _strnicmp
+#define strtok_r     strtok_s
+#define popen        _popen
+#define pclose       _pclose
+
+inline tm* localtime_r(const time_t* tin, tm* tout) {
+  if (!localtime_s(tout, tin)) return tout;
+
+  return nullptr;
+}
+
+#endif