Fix a hang where ctrl-d can hang the next call to Readline

[tpodowd]

When running the examples/readline-demo, you can enter the sleep command which causes the loop to sleep for 4 seconds before it calls Readline() again. If you press ctrl-d during the sleep, the program will hang.

Normally ctrl-d is mapped to CharDelete (rune:4), however termios manpage states that when not in raw mode the following is true:

VEOF (004, EOT, Ctrl-D) End-of-file character (EOF). More
precisely: this character causes the pending tty buffer to be sent
to the waiting user program without waiting for end-of-line. If
it is the first character of the line, the read(2) in the user
program returns 0, which signifies end-of-file. Recognized
when ICANON is set, and then not passed as input.

Between calls to Readline, the terminal is not in raw mode and thus read returns 0. Note that err=io.EOF is correctly unset.

Previously the terminal ioloop returned the 0 on the t.outchan which is being read by the operation ioloop. Unfortunately, 0 is also used to indicate EOF and thus the operation ioloop terminates which means that nobody will read from t.outchan any more and thus input processing stops and causes the hang.

The fix is simply to ignore 0 in the terminal and go on expecting characters.