(get_device_once): add nomesg argument.  chmod slave tty to 0600
  if nomesg.  more error tests.
  (no_mesg): new function.
  (pty_open): make slave tty's mode 0600.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@20795 b2dd03c8-39d4-4d8f-98ff-823fe69b080e