Add "archive" as an alias for "archive-z2"

I find the "-z2" is really a long ago relic of the past when I changed
the format.  We no longer have anything to do with the original
`archive`, so let's start allowing people to type `--mode=archive`
which just looks saner.

At some point later I'll update the docs too, but it'll be an annoying
transition period as we'll have to say "On older OSTree, use -z2" etc.

Closes: #346
Approved by: giuseppe

tests/basic-test.sh

@@ -278,7 +278,7 @@ echo "ok prune"
 
 cd ${test_tmpdir}
 rm repo3 -rf
-${CMD_PREFIX} ostree --repo=repo3 init --mode=archive-z2
+${CMD_PREFIX} ostree --repo=repo3 init --mode=archive
 ${CMD_PREFIX} ostree --repo=repo3 pull-local --remote=aremote repo test2
 rm repo3/refs/remotes -rf
 mkdir repo3/refs/remotes
@@ -405,7 +405,7 @@ echo "ok commit of fifo was rejected"
 cd ${test_tmpdir}
 rm repo2 -rf
 mkdir repo2
-${CMD_PREFIX} ostree --repo=repo2 init --mode=archive-z2
+${CMD_PREFIX} ostree --repo=repo2 init --mode=archive
 ${CMD_PREFIX} ostree --repo=repo2 pull-local repo
 rm -rf test2-checkout
 ${CMD_PREFIX} ostree --repo=repo2 checkout -U --disable-cache test2 test2-checkout