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_socket.cc

@@ -247,7 +247,7 @@ fhandler_socket::fstat (struct stat *buf, path_conv *pc)
   return fh.fstat (buf, pc);
 }
 
-int
+int __stdcall
 fhandler_socket::read (void *ptr, size_t len)
 {
   sigframe thisframe (mainthread);