Do not run (or allow to disable) motd updates in non-interactive sessions

[cpaelzer]

Hi,
we love the pam_motd feature that was discussed and implmented as part of #47.
It is in heavy use in Ubuntu, so heavy that I recently had to cut down some of the things executed a bit to not slow down login too much :-)
Now everything is either fast or caches results as recommended which is much better, but not perfect.

Out of working on that bug a question came up which is: "Wouldn't it be better to not execute (or make if configurable to behave that way) those motd scripts at all if the login that is processed by pam is non-interactive?"
After all within a non-interactive session we can assume that there will be nobody that is able to consume/perceive the messages that we generate.

I see a few checks for interactivity in conf/pam_conv1/pam_conv_l.c and doc/specs/parse_l.c but none seems directly applicable/reusable for modules/pam_motd/pam_motd.c. Due to that and since I wasn't sure how the stance to this would be in general I didn't yet try to implement it yet. Instead filed this issue to check how you all generally think about it and if there might be any suggestions how/where to best implement it.

P.S. If I'm just failing to see a little tweak to pam.conf* that would already make it behave that way I'm equally happy to learn about that instead :-)