Comment 16 for bug 525674

Here the bigger, but more architecturally complete solution for wily&upstream (which is actually identical at this time).

Summary:
- move the actual updating part out of the pam based trigger completely (avoids slowdown)
- pam based motd now only prints the cached info (if existing)
- hook into apt with APT::Update::Post-Invoke-Success to update the cached file (thanks mvo and pitti for that good IRC discussion)
- removed one level of indirection (apt hook -> stamp file -> on pam login check stamp -> update) to the more separate
  (apt hook -> update cached info) (on pam login -> print cached info)
- while the chance of a concurrent update is now (almost) impossible it keeps the atomic cache update and cleanup to be sure

It was working find in all my builds and tests on wily.
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