Return at most one line of input in canonical mode

Return at most one line of input in canonical mode

'man termios' says:
"A read(2) returns at most one line of input" in canonical mode.

On cygwin 2.5.1, read(2) returns all data in buffer if the buffer
size specified is large enough. This behaviour is correct in
noncanonical mode, but is not correct in canonical mode.

While checking this problem, I found a bug of tcflush().  tcflush()
flushes only partial data in the buffer.  The patch also fixes this bug.

The patch has also been tested against the problem reported in
https://cygwin.com/ml/cygwin/2016-05/msg00318.html.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>