Configuration error running su in unstable

[Chris Lale <chrislale@untrammelled.co.uk>]

Since upgrading unstable a week or two ago, running su from a terminal 
reports these errors:

configuration error - unknown item 'QUOTAS_ENAB' (notify administrator)
configuration error - unknown item 'NOLOGIN_STR' (notify administrator)
configuration error - unknown item 'ENV_HZ' (notify administrator)
configuration error - unknown item 'PASS_MAX_LEN' (notify administrator)
configuration error - unknown item 'CHFN_AUTH' (notify administrator)
configuration error - unknown item 'CLOSE_SESSIONS' (notify administrator)

The root prompt appears and is useable. However, gksu fails to run 
applications such as synaptic:

Failed to run synaptic as user root:
Failed to communicate with gksu-run-helper.
Received:
configuration error - unknown item 'QUOTAS_ENAB' (notify administrator)

Some cron scripts also mail errors to root. Further upgrades have failed 
to fix the problem. I currently have login 1:4.0.12-6 (the package which 
provides su). I would be grateful for any suggestions.

Chris.