Add some awareness of short file names

- Added I_GetLongPathName(). It wraps the Win32 API's GetLongPathName().
  DArgs::CollectFiles() now calls this for every argument it processes, so
  any arguments passed using short file names will be converted to long
  file names. This is mainly of interest so that savegames will never
  record the short file name, which can change based on what else is in
  the directory.

src/win32/i_system.cpp

@@ -1569,3 +1569,31 @@ unsigned int I_MakeRNGSeed()
 	CryptReleaseContext(prov, 0);
 	return seed;
 }
+
+//==========================================================================
+//
+// I_GetLongPathName
+//
+// Returns the long version of the path, or the original if there isn't
+// anything worth changing.
+//
+//==========================================================================
+
+FString I_GetLongPathName(FString shortpath)
+{
+	DWORD buffsize = GetLongPathName(shortpath.GetChars(), NULL, 0);
+	if (buffsize == 0)
+	{ // nothing to change (it doesn't exist, maybe?)
+		return shortpath;
+	}
+	TCHAR *buff = new TCHAR[buffsize];
+	DWORD buffsize2 = GetLongPathName(shortpath.GetChars(), buff, buffsize);
+	if (buffsize2 >= buffsize)
+	{ // Failure! Just return the short path
+		delete[] buff;
+		return shortpath;
+	}
+	FString longpath(buff, buffsize2);
+	delete[] buff;
+	return longpath;
+}