Don't use cached passwords if "extauth_cache: 0"

Regarding "extauth_cache", the guide says: "The integer 0 (zero) enables
caching for statistics, but doesn't use that cached information to
authenticate users."  Make sure the cached password isn't used even if
the user is currently logged in with another resource.
Merge pull request #217 from weiss/fix-extauth-cache-usage

Don't use cached passwords if "extauth_cache: 0"