libostree: Allow compression level to be set for archive-z2 stream

Add a ostree_raw_file_to_archive_z2_stream_with_options() variant of
ostree_raw_file_to_archive_z2_stream(), to allow a compression-level
option to be passed in and passed through to zlib.

This is useful when building archive-z2 files on the fly for
transmission over a non-bandwidth-limited channel, such as a local
network. In this case, CPU time is more valuable than bandwidth, so we
want a low compression level.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #721
Approved by: cgwalters

src/libostree/libostree.sym

@@ -381,12 +381,10 @@ global:
  *                         NOTE NOTE NOTE
  */
 
-/* Uncomment this when adding the first new symbol for 2
-LIBOSTREE_2017.XX {
+LIBOSTREE_2017.3 {
 global:
-	someostree_symbol_deleteme;
+        ostree_raw_file_to_archive_z2_stream_with_options;
 } LIBOSTREE_2017.2;
-*/
 
 /* Stub section for the stable release *after* this development one; don't
  * edit this other than to update the last number.  This is just a copy/paste