add support for pam_u2f to nixos pam module

This adds support for authenticating using a U2F device such as a
yubikey neo.

pkgs/os-specific/linux/pam_u2f/default.nix

@@ -0,0 +1,22 @@
+{ pkgs, fetchurl, stdenv }:
+stdenv.mkDerivation rec {
+  name    = "pam_u2f-${version}";
+  version = "0.0.1";
+  src     = fetchurl {
+    url = "https://developers.yubico.com/pam-u2f/Releases/${name}.tar.gz";
+    sha256 = "0p1wia4nfw5h0pmy1lcgwsbrlm7z39v1n37692lgqfzyg1kmpv7l";
+  };
+  buildInputs = with pkgs; [ asciidoc autoconf automake docbook_xml_dtd_45 libtool libu2f-host libu2f-server libxml2 libxslt pkgconfig pam ];
+
+  installFlags = [
+    "PAMDIR=$(out)/lib/security"
+  ];
+
+  meta = with stdenv.lib; {
+    homepage = https://developers.yubico.com/pam-u2f/;
+    description = "A PAM module for allowing authentication with a U2F device";
+    license = licenses.bsd2;
+    platforms = platforms.unix;
+    maintainers = with maintainers; [ philandstuff ];
+  };
+}