tty: better fix for Bug#26371

tty: better fix for Bug#26371

* src/tty.c (TTY_USAGE): Rename from TTY_FAILURE, since this
is used only for usage failures.  All uses changed.
(TTY_TTYNAME_FAILURE): New constant.
(main): Remove no-longer-needed assignment of ENOENT to errno.
Make status-setting clearer too.
Report an error if ttyname fails even though stdin is a terminal,
instead of silently pretending that stdin is not a terminal.
* tests/tty/tty.sh: Test for this issue.  This should fix Bug#78244.