Factor out Windows utility function get_system_dir().

The code to find out the location of the c:\windows\system32 directory
was already present, in load_system32_dll(). Now it's moved out into a
function of its own, so it can be called in other contexts.

windows/platform.h

@@ -566,6 +566,7 @@ HWND event_log_window(void);
 extern DWORD osMajorVersion, osMinorVersion, osPlatformId;
 void init_winver(void);
 void dll_hijacking_protection(void);
+const char *get_system_dir(void);
 HMODULE load_system32_dll(const char *libname);
 const char *win_strerror(int error);
 void restrict_process_acl(void);