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_tty.cc

@@ -619,7 +619,7 @@ fhandler_tty_slave::write (const void *ptr, size_t len)
   return towrite;
 }
 
-int
+int __stdcall
 fhandler_tty_slave::read (void *ptr, size_t len)
 {
   DWORD n;
@@ -1032,7 +1032,7 @@ fhandler_pty_master::write (const void *ptr, size_t len)
   return len;
 }
 
-int
+int __stdcall
 fhandler_pty_master::read (void *ptr, size_t len)
 {
   return process_slave_output ((char *) ptr, len, pktmode);