tree-wide: Replace archive-z2 with archive

In almost all places. There are just a few exceptions; one tricky bit for
example is that the repo config must still have `mode=archive-z2`, since
`archive` used to mean something else. (We could very likely just get rid of
that check, but eh, later).

I also added a test that one can still do `ostree repo init --mode=archive-z2`.

Closes: #1125
Approved by: jlebon

src/libostree/ostree-repo-static-delta-processing.c

@@ -608,7 +608,7 @@ dispatch_open_splice_and_close (OstreeRepo                 *repo,
         }
       else
         {
-          /* Slower path, for symlinks and unpacking deltas into archive-z2 */
+          /* Slower path, for symlinks and unpacking deltas into archive */
           g_autoptr(GFileInfo) finfo =
             _ostree_mode_uidgid_to_gfileinfo (state->mode, state->uid, state->gid);