Ensure that all fhandler_*::read definitions are __stdcall throughout.

* fhandler.cc (fhandler_base::set_inheritance): Be more defensive in debugging
code.
* fhandler.h: Adjust regparms throughout to reflect passing 'this' parameter.
* fhandler_console.cc (fhandler_console::read): Remove unneeded test.  Only
honor "key down" events.
* miscfuncs.cc (strcasestr): Reorganize for efficient code use.
(check_null_empty_str_errno): Ditto.
(__check_null_invalid_struct_errno): Ditto.
(__check_invalid_read_ptr_errno): Ditto.
* syscalls.cc (_read): Return 0 when length == 0, as per Single UNIX
Specification.

winsup/cygwin/fhandler_windows.cc

@@ -79,7 +79,7 @@ fhandler_windows::write (const void *buf, size_t)
     return SendMessage (ptr->hwnd, ptr->message, ptr->wParam, ptr->lParam);
 }
 
-int
+int __stdcall
 fhandler_windows::read (void *buf, size_t len)
 {
   MSG *ptr = (MSG *) buf;