Cygwin: pty: add pseudo console support.

- Support pseudo console in PTY. Pseudo console is a new feature
  in Windows 10 1809, which provides console APIs on virtual
  terminal. With this patch, native console applications can work
  in PTYs such as mintty, ssh, gnu screen or tmux.

winsup/cygwin/init.cc

@@ -100,6 +100,7 @@ dll_entry (HANDLE h, DWORD reason, void *static_load)
 	 will always fall back to __global_locale, rather then crash due to
 	 _REENT->_locale having an arbitrary value. */
       alloca_dummy = alloca (CYGTLS_PADSIZE);
+      ZeroMemory (alloca_dummy, CYGTLS_PADSIZE);
       memcpy (_REENT, _GLOBAL_REENT, sizeof (struct _reent));
 
       dll_crt0_0 ();