[dwisiswant0]

GetMode wiped the terminal attributes (iflag, oflag, cflag, lflag and
the control characters, so echo, canonical mode and signals) and
returned a non-nil "errno 0" error on darwin and linux.

The exported TCGETS and TCSETS values were assigned crosswise in
values_darwin.go and values_linux.go, so getTermios issued the "set"
ioctl with a zeroed Termios and setTermios issued the "get" ioctl. On
top of that, the raw syscall's Errno was returned as error without
checking for zero, which makes every success a non-nil error. The
posix helpers and the windows ones also ignored their *os.File argument
and always operated on os.Stdin.

Assign each constant its matching request, return nil when the errno is
zero, and use the file the caller passes. On posix, GetMode now returns
a nil interface on failure so a failed call cannot be passed on to
SetRawMode as a valid mode; the windows implementation still returns
the zero mode with its error.

Because the mode is nil after a failed GetMode, KeyPressReader would
have reported the opaque "invalid syscall.Termios" for a stdin that is
not a terminal, where it used to surface the errno. Start in raw mode
now returns GetMode's error, and Stop does nothing when Start never
obtained a mode, since it changed nothing to restore.

The posix ioctl and read errors, and the GetMode error returned by
Start, are now wrapped with %w and a short description of the failed
operation, so errors.Is and errors.As still match the underlying errno.

The tests drive a pseudo-terminal pair and swap os.Stdin for a second
one with different attributes, so they need no controlling terminal and
notice any call that goes to stdin instead of the given file.

Fixes: #831