libotutil: Add ot_keyfile_copy_group()

Copies all the keys of a group from one GKeyFile to another.
repo: Add an internal struct to manage remotes

OstreeRemote is a reference-counted struct that encompasses data about a
remote, whether read from a configuration file or created explicitly via
ostree_repo_remote_add().

OstreeRemotes are held in an internal table indexed by remote name.
This solves some problems caused by merging system-wide remote data into
the OstreeRepo's internal config key file.

Also fixes https://bugzilla.gnome.org/show_bug.cgi?id=740911
repo: Add ostree_repo_remote_get_url()

Peeking at remote details by way of ostree_repo_copy_config() doesn't
work anymore.
repo: Add private functions to access remote options

ostree_repo_pull_with_options() needs this, and I'd rather keep the
OstreeRemote struct definition tucked away in ostree-repo.c with its
own internal API.