[ 924301 ] A leak case with cmd.py & readline & exception

by ensuring that the readline completion function is always reset
even in the case of an exception being raised.  As a bonus, this
makes the documentation for pre & postloop accurate again.

This patch adds stdin, stdout as optional arguments to the cmd.Cmd
constructor (defaulting to sys.stdin, sys.stdout), and changes the Cmd
methods throughout to use self.stdout.write() and self.stdin.foo for
output and input. This allows much greater flexibility for using cmd -
for instance, hooking it into a telnet server.

Patch for library module and for documentation.

[ 676342 ] after using pdb readline does not work correctly

using Michael Stone's patch so the completer functionality of
cmd is only setup between preloop and postloop.

This was introduced in 1998 in rev. 1.13, where I imported extensive
patches that, I am sad to say, I didn't review as carefully as I
should have.