Ability to check if user account is not expired/invalid

[raphendyr]

There is sometimes need to check if user could authenticate (check if user account is not expired). For example in gitlab when user connects using key over ssh and we want to validate that user account tied to the key is still valid in our environment.

For this the second command pam_acct_mgmt in rpam_ext.c is used for authenticated user, but it would be nice to be able to call this function without password. Thus another method like rpam.valid(username) could be added.

[canweriotnow]

I'm not too active on this project anymore but if you'd like to send a pull request I'd be happy to consider it.