add config to run alternative git-annex-shell program on remote

    execute `remote.<name>.annex-shell` on remote, if set
    
    It is useful to be able to specify an alternative git-annex-shell
    program to execute on the remote, e.g., to run a version not on the
    PATH.  Use `remote.<name>.annex-shell` if specified, instead of the
    default "git-annex-shell" i.e., first thus named executable on the
    PATH.