Add borg backup software (fork of attic)

Borg is a fork of attic backup software. It is more actively developed,
with more features.

pkgs/tools/backup/borg/default.nix

@@ -0,0 +1,27 @@
+{ stdenv, fetchzip, python3Packages, openssl, acl }:
+
+python3Packages.buildPythonPackage rec {
+  name = "borg-${version}";
+  version = "0.23.0";
+  namePrefix = "";
+
+  src = fetchzip {
+    name = "${name}-src";
+    url = "https://github.com/borgbackup/borg/archive/${version}.tar.gz";
+    sha256 = "1ns00bhrh4zm1s70mm32gnahj7yh4jdpkb8ziarhvcnknz7aga67";
+  };
+
+  propagatedBuildInputs = with python3Packages;
+    [ cython msgpack openssl acl llfuse tox detox ];
+
+  preConfigure = ''
+    export BORG_OPENSSL_PREFIX="${openssl}"
+  '';
+
+  meta = with stdenv.lib; {
+    description = "A deduplicating backup program (attic fork)";
+    homepage = https://borgbackup.github.io/;
+    license = licenses.bsd3;
+    platforms = platforms.unix; # Darwin and FreeBSD mentioned on homepage
+  };
+}

pkgs/top-level/all-packages.nix

@@ -755,6 +755,8 @@ let
 
   bochs = callPackage ../applications/virtualization/bochs { };
 
+  borg = callPackage ../tools/backup/borg { };
+
   boomerang = callPackage ../development/tools/boomerang { };
 
   boost-build = callPackage ../development/tools/boost-build { };