libostree: Make OstreeLibarchiveInputStream explicitly private

We weren't installing the headers, but at the moment all symbols
starting with ostree_ were being exported.  Fix that by prefixing
non-static symbols with '_'.

https://bugzilla.gnome.org/show_bug.cgi?id=731369
libostree: Make OstreeTlsCertInteraction explicitly private

Similar to previous commit, this should be private.

https://bugzilla.gnome.org/show_bug.cgi?id=731369
libostree: Make OstreeFetcher explicitly private

Per previous commit.  We're not attempting to export an HTTP to
clients, the high level API is ostree_repo_pull().

https://bugzilla.gnome.org/show_bug.cgi?id=731369