login-utils: Enable building util-linux against OpenPAM

OpenPAM is compatible with util-linux, with a few changes, namely
using OpenPAM's conversation function, openpam_ttyconv.

We check for Linux-PAM by querying for security/pam_misc.h, and OpenPAM
by querying for security/openpam.h.

Signed-off-by: Will Johansson <will.johansson@gmail.com>