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/syscalls.cc

@@ -287,6 +287,9 @@ setsid (void)
 extern "C" ssize_t
 _read (int fd, void *ptr, size_t len)
 {
+  if (len == 0)
+    return 0;
+
   if (__check_null_invalid_struct_errno (ptr, len))
     return -1;