Build structure for contribified Linux-PAM, plus some home-grown

Build structure for contribified Linux-PAM, plus some home-grown
modules for FreeBSD's standard authentication methods.  Although
the Linux-PAM modules are present in the contrib tree, we don't
use any of them.

The main library "libpam" is composed of sources taken from three
places.  First are the standard Linux-PAM libpam sources from the
contrib tree.  Second are the Linux-PAM "libpam_misc" sources, also
from the contrib tree.  In Linux these form a separate library.
But as Mike Smith pointed out to me, that seems pointless, so I
have combined them into the libpam library.  Third are some additional
sources from the "src/lib/libpam" tree with some common functions
that make it easier to write modules.  Those I wrote myself.

This work has been donated to FreeBSD by Juniper Networks, Inc.