libostree: Add g_autoptr() support for the public types

libostree: Add g_autoptr() support for the public types

To avoid breaking the ostree users we hide the autoptr setup behind an
OSTREE_WITH_AUTOCLEANUPS variable. The variable is set to 0 by
default, so users that want to start using them, have to define the
variable themselves to 1.

Probably after some transition period, the variable will be set 1 by
default.

This is conditional on GLib >= 2.44.0 being available. It does not
bump ostree's dependency on GLib.

Closes: #295
Approved by: cgwalters