feat: server-side tolerance for unknown --info tokens (upstream parity)

[oferchen]

Summary

Make oc-rsync's --info= parser tolerate unknown tokens when running on the server side, matching upstream rsync's parse_output_words() (options.c:465). The client-side parser keeps erroring on unknown tokens so typos still surface at the source.

This preserves cross-version compatibility: a newer client can forward info names this build has not learned yet without breaking the transfer.

Upstream reference

// rsync-3.4.1/options.c:465-469 - parse_output_words()
if (len && !words[j].name && !am_server) {
    rprintf(FERROR, "Unknown %s item: \"%.*s\"\n",
        words[j].help, len, str);
    exit_cleanup(RERR_SYNTAX);
}

The !am_server clause is the entire mechanism: known tokens still apply on either side, but the Unknown ... exit only fires on the client.

Changes

• parse_info_flags_server() is a new entry point that calls the shared parser with am_server=true; InfoFlagSettings::apply_with_mode() skips the unknown-token error in that mode while still rejecting malformed inputs (empty tokens, out-of-range levels for known names).
• Server-mode argument parsing now recognises --info=FLAGS, captures the raw values, and validates them via parse_info_flags_server. Before this change the argument leaked into the positional path list because is_known_server_long_flag did not know about it.

Test plan

• parse_info_flags_client_rejects_unknown_token - client mode still errors with Unknown --info item ...
• parse_info_flags_server_accepts_unknown_token - server mode silently accepts an unknown token
• parse_info_flags_server_mixes_known_and_unknown - known tokens still apply in server mode when unknown ones are interleaved
• long_flags_info_is_captured - server-mode argument parser captures --info=FLAGS instead of dropping it into positional args

Refs #2168