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-core.c

@@ -312,7 +312,7 @@ _ostree_file_header_new (GFileInfo         *file_info,
  * @file_info: a #GFileInfo
  * @xattrs: (allow-none): Optional extended attribute array
  *
- * Returns: (transfer full): A new #GVariant containing file header for an archive-z2 repository
+ * Returns: (transfer full): A new #GVariant containing file header for an archive repository
  */
 GVariant *
 _ostree_zlib_file_header_new (GFileInfo         *file_info,