clap: Use ArgAction::Append

clap: Use ArgAction::Append

Gets rid of this 3 to 4 deprecation warning.

The warning's suggestion to use "multiple_occurrences" seems wrong
since that is also deprecated ?!  It links to Arg::action, and
emprically this construction DTRT.

    warning: use of deprecated method `clap::Arg::<'help>::multiple`: Split into `Arg::multiple_occurrences` (most likely what you want)  and `Arg::multiple_values`
      --> src/config.rs:89:16
       |
    89 |   #[clap(long, multiple=true, number_of_values=1)]
       |                ^^^^^^^^
       |
       = note: `#[warn(deprecated)]` on by default

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>