Vendor raptor source files (was embedded gitlink)

Tree vendored at upstream 0019c5dd; origin + license recorded in
raptor/VENDOR.txt. Windows SDL dev packages excluded via .gitignore.

raptor/src/help.h

@@ -0,0 +1,4 @@
+#pragma once
+
+void HELP_Init(void);
+void HELP_Win(const char *strpage);