Add an export builtin, and API to write to libarchive

At the moment I'm looking at using rpm-ostree to manage RPM inputs
which can then be converted into Docker images.  It's most convenient
if we can stream directly out of libostree rather than doing a
checkout + tar combination.

There are also backup/debugging etc. reasons to implement `export` as
well.

tests/test-export.sh

@@ -0,0 +1,50 @@
+#!/bin/bash
+#
+# Copyright (C) 2016 Colin Walters <walters@verbum.org>
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the
+# Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+# Boston, MA 02111-1307, USA.
+
+set -euo pipefail
+
+. $(dirname $0)/libtest.sh
+
+setup_test_repository "archive-z2"
+
+echo '1..2'
+
+$OSTREE checkout test2 test2-co
+$OSTREE commit --no-xattrs -b test2-noxattrs -s "test2 without xattrs" --tree=dir=test2-co
+rm test2-co -rf
+
+cd ${test_tmpdir}
+${OSTREE} 'export' test2-noxattrs -o test2.tar
+mkdir t
+(cd t && tar xf ../test2.tar)
+ostree --repo=repo diff --no-xattrs test2-noxattrs ./t > diff.txt
+assert_file_empty diff.txt
+rm test2.tar diff.txt t -rf
+
+echo 'ok export gnutar diff (no xattrs)'
+
+cd ${test_tmpdir}
+${OSTREE} 'export' test2 -o test2.tar
+${OSTREE} commit -b test2-from-tar -s 'Import from tar' --tree=tar=test2.tar
+ostree --repo=repo diff test2 test2-from-tar
+assert_file_empty diff.txt
+rm test2.tar diff.txt t -rf
+
+echo 'ok export import'
+