make-dbus-conf: Don't try to access network

This is the output of the builder:

building path(s) `/nix/store/khkcfb8433i9mabb6wnb8ik6p9skg644-dbus-1'
error : connection refused
error : connection refused

However, even when using --nonet we'd still get this:

I/O error : Attempt to load network entity
http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd



So in order to avoid this, we now provide an XML catalog file, mapping
the public URLs to the local DTD paths inside the store instead of using
--path (which doesn't seem to work with xsltproc).

Tested this by comparing the SHA256 (nix-hash --type sha256) of the
output path generated by:

nix-build -E '(import ./. {}).makeDBusConf {
  suidHelper = "SUIDHELPER";
  serviceDirectories = [ "SERVICEDIR1" "SERVICEDIR1" ];
}'

... with the SHA256 of the generated output path prior to this commit
and they have the same hash:

6f3f9594b12fddbff9407b85252b6f649da11f56b7fd514f761966c11399a7ab

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @abbradar