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-pull.c

@@ -3631,7 +3631,7 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
 
     }
 
-  /* We can't use static deltas if pulling into an archive-z2 repo. */
+  /* We can't use static deltas if pulling into an archive repo. */
   if (self->mode == OSTREE_REPO_MODE_ARCHIVE_Z2)
     {
       if (pull_data->require_static_deltas)