feature: use database to hold secret instead of file

[flowers9]

So, I already have a modified version of google-authenticator.c, pam_google_authenticator.c, and pam_google_authenticator_unittest.c that can connect to a postgres database (and have a working case with a database on one machine and ssh login with 2fa on another). I've written some documentation on how to compile them, and on how to set up a postgres database for them to access.

If I were to write a PR to add this to, say, /contrib/postgres, (that is, the three modified files and related documentation) is that something you would be interested in adding to the repository?

[ThomasHabets]

With the level of general activity on this project, I'm not sure it would be a feature best hosted here. It may be better off as a fork on github.