[fixup] repo: Add an internal struct to manage remotes

Ensure a consistent "remote not found" error message.
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.
Improve "ostree remote" help output

Must have glossed over these because the commands are so simple.

 - List subcommands for "ostree remote --help".

 - Only show options relevant to COMMAND for
   "ostree remote COMMAND --help".