provide --help option

[flokli]

WIth #18 applied, I successfully built jmap-server, and tried running it:

❯ result/bin/stalwart-jmap
2022-11-20T13:41:13.997795Z  WARN stalwart_jmap: Warning: Hostname parameter 'jmap-url' was not specified, using 'http://localhost:8080'.
Failed to open database: Internal error: Failed to create index directory /usr/local/stalwart-jmap/data/idx: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }

I tried the --help option to see available options, but it seems it's not rendering help:

❯ result/bin/stalwart-jmap --help
2022-11-20T13:39:38.129059Z  WARN stalwart_jmap: Warning: Hostname parameter 'jmap-url' was not specified, using 'http://localhost:8080'.
Failed to open database: Internal error: Failed to create index directory /usr/local/stalwart-jmap/data/idx: Os { code: 13, kind: PermissionDenied, message: "Permission denied" }

[mdecimus]

Hi @flokli , Stalwart JMAP v0.3 was just released and it supports only --config as a parameter. If this parameter is missing it prints out a message.