pam: Use freebsd.pam on FreeBSD

FreeBSD pam is based on openpam, but is not exactly the same.
Use FreeBSD pam for better compatibility with existing FreeBSD
configurations and code.

pkgs/top-level/all-packages.nix

@@ -12572,7 +12572,13 @@ with pkgs;
     inherit (darwin.apple_sdk.frameworks) Security;
   };
 
-  pam = if stdenv.hostPlatform.isLinux then linux-pam else openpam;
+  pam =
+    if stdenv.hostPlatform.isLinux then
+      linux-pam
+    else if stdenv.hostPlatform.isFreeBSD then
+      freebsd.libpam
+    else
+      openpam;
 
   # pam_bioapi ( see http://www.thinkwiki.org/wiki/How_to_enable_the_fingerprint_reader )