lib/remote: Store name of remote providing keyring for dynamic remotes

lib/remote: Store name of remote providing keyring for dynamic remotes

When pulling from a dynamic (peer to peer) remote, the remote’s name is
set to a unique, generated string which doesn’t exist in repo/config. If
doing a non-mirror pull, however, we don’t want to use this name in the
refspecs for newly created or updated refs — we want to use the name of
the remote which provided the keyring for the pull (this will be a
remote from repo/config whose collection ID matches that being used for
the peer to peer pull).

Store both names in OstreeRemote. The name to use for refspecs is stored
as refspec_name, and is typically NULL unless it differs from name.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1202
Approved by: cgwalters