login-utils: Enable building util-linux against OpenPAM

[Will Johansson <will.johansson@gmail.com>]

Hello,

I recently experimented with cross-compiling for embedded devices, and
sought to try out util-linux with OpenPAM instead of Linux-PAM because
OpenPAM was simpler to cross-compile. Unfortunately for me, it did not
work out of the box, so I decided to try to patch util-linux to support
compiling against OpenPAM. Turned out to be a fairly simple exercise.

This is the fruit of my efforts. There is a caveat, which I will
be happy to address if it's a problem. The patch's configuration does
not support building if you happen to have both Linux-PAM and OpenPAM
in your build paths. I could update it to prefer Linux-PAM by default.
I'm not sure how it'd would work if OpenPAM libraries is also in the
search path, as -lpam is used by both Linux-PAM and OpenPAM (probably
undefined behavior).

Either way, it should compile fine against Linux-PAM and OpenPAM.
There are some unused variables in login.c, but seems to have to do
with _HAVE_UT_TV, which is not the case in my configuration. I'm
more than happy to provide a separate patch to wrap the unused
variables with an #ifdef _HAVE_UT_TV.

Warm regards,

Will Johansson