clap: use value_parser

clap: use value_parser

Removes a deprecation warning (this is recommended by the clap 3 to 4
migration guide).

    warning: use of deprecated function `<reporter::LogOpts as clap::Args>::augment_args::parse_try_from_str`: Replaced with `#[clap(value_parser = ...)]`
      --> src/reporter.rs:22:35
       |
    22 |   #[clap(long, parse(try_from_str=parse_syslog_facility))]
       |                                   ^^^^^^^^^^^^^^^^^^^^^
       |
       = note: `#[warn(deprecated)]` on by default

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