Correct the interest masks in the Tcl_CreateFileHandler() calls in
PipeWatchProc().  When we are interested in both readable and writable
events of a command pipeline channel, we only want the readable from
the read end of the pipe, and the writable from the write end of the pipe.
Correct the interest masks in the Tcl_CreateFileHandler() calls in PipeWatchProc(). When we are interested in both readable and writable events of a command pipeline channel, we only want the readable from the read end of the pipe, and the writable from the write end of the pipe.