echoing of stdin

[Ross Paterson]

Hugs currently turns off echoing of stdin before evaluation, and turns
it on again afterwards.  In partial compensation, getChar echoes chars
to stdout (which is wierd if either stdin or stdout is redirected)
but getContents doesn't.

Is there any reason to retain this behaviour, rather than treating
stdin like any other handle?  For applications that don't want echoing,
we could provide implementations of the System.IO functions hSetEcho,
hGetEcho and hIsTerminalDevice.