repo: Use constructed, not constructor

It's simpler to do this after the object is constructed, rather
than in the middle of construction.

https://bugzilla.gnome.org/show_bug.cgi?id=707582
repo: Move the 'init' builtin command to a public API, ostree_repo_create

This continues the goal of making more of ostree accessible by API,
rather than forking out to subprocesses.

https://bugzilla.gnome.org/show_bug.cgi?id=707582
repo: Rename ostree_repo_check to ostree_repo_open

As it more clearly describes what the function does: load the
repo from disk and initialize it.

At the same time, add a cancellable parameter.

https://bugzilla.gnome.org/show_bug.cgi?id=707582