Use PAM properly, which allows us to handle other auth types

Previously I was just presenting a static username/password box, and
then running PAM with pre-set credentials.  This works just fine when
PAM is expecting a username and password, but if it's expecting
something like a fingerprint scan or a hardware security token, this
wouldn't entirely work right.  Well, it would "work", but the
username/password dialog would be displayed, and then hitting "Unlock"
would start a different auth process with no visible feedback as to
what's supposed to happen.

This also means I need to switch PAM wrapper crates; the one I was using
before did not allow passing a fixed username to the underlying
pam_start() call, which meant that PAM would try to prompt the user for
it, which is not what we want.

debian/control

@@ -14,6 +14,7 @@ Build-Depends:
   libpam0g-dev,
   cargo (>= 0.57),
   rustc (>= 1.58),
+  clang,
   help2man
 
 Package: bscreensaver