BUG #1418: RFC: Challenge/response authentication support

The following bug has been logged online:

Bug reference:      1418
Logged by:          Craig Ruff
Email address:      cruff@ucar.edu
PostgreSQL version: 8.0
Operating system:   Any
Description:        RFC: Challenge/response authentication support
Details:

PAM supports challenge response authentication.  It is desirable that psql
and the backend support this by displaying the PAM conversation routine
message(s) and returing a response (optionally echoed to the user).

I had a look at the code, and the backend support isn't too bad, but psql
itself does not appear to be structured in a way to handle this easily.  The
current method of just closing the backend connection, prompting for the
password and trying again does not work since the one-time password
challenge/response method is stateful.

Unfortunately, at the moment, I don't have the time to delve into fixing
this up further, but thought I'd let the list know in case someone else is
hacking on psql.