user.c expiry support

[veins <veins@skreel.org>]

I have been dealing with a lot of problems to get accounts expiry to work,
it is really a pain in the ass and after watching the sources i have seen
a strange behaviour.

how comes that the date has to be entered in format %c to be converted to
format %s right after ?  Wouldn't have been easier to use strftime() in
place of strptime() ?  Anyways, I wrote a small patch that makes user.c
use strftime() instead of the user and wanted to know if there would be
some repercussion of making that before I actually use it on the production
system. My problem is that i'm having a lot of problems to get the other
way working like I want and that it seems weird that we have to enter the
%c format to get user.c output in %s when it's easier to enter in %s format.

Thanks,
veins