Add support for pulling from remote archives

This necessitated a large set of changes.

We now support an "archive" mode for repositories.  In this mode,
files are stored "packed" rather than hard linked.  This allows one to
e.g. store an OSTree repository with root-owned files as non-root.  It
is also used as the basis for serving repositories via HTTP.

While doing this I realized that GVariant is endianness-dependent; I
decided to just store all data in big endian.

tests/.gitignore

@@ -1 +1,4 @@
 !Makefile
+ostree-http-server
+run-apache
+tmpdir-lifecycle