bug#18506: stty dsusp undef fails.

[Edward Huff]

$ stty dsusp undef
stty: invalid argument ‘dsusp’
Try 'stty --help' for more information.
$ stty --help | grep dsusp
 * dsusp CHAR    CHAR will send a terminal stop signal once input flushed
$ stty --version
stty (GNU coreutils) 8.21
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by David MacKenzie.
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