s command reads the next line from <I>channelId</I>, returns everything
in the line up to (but not including) the end-of-line character(s), and
discards the end-of-line character(s).
<P>
<I>ChannelId</I> must be an identifier for an open channel such as the
Tcl standard input channel (<B><A HREF="../TclLib/GetStdChan.html">stdin</A></B>), the return value from an
invocation of <B><A HREF="open.html">open</A></B> or <B><A HREF="socket.html">socket</A></B>, or the result of a channel
creation command provided