Add cmdline option for rspamd Settings-ID HTTP header

[mneumann]

This allows to distinguish between multiple rspamd settings
by using separate opensmtpd filters with different -settings-id parameters.

For example:

dkim {
	id = "dkim";
	apply {
		groups_enabled = ["dkim"];
	}
}

filter rspamd-dkim proc-exec "opensmtpd-filter-rspamd -url http://localhost:11333 -settings-id dkim"

[mikejsavage]

I ran into this a few days ago, so specifically the issue is that you want outgoing mail to go through rspamd for dkim signing but not any spam checking, because sending mails with X-Spam: yes headers is awkward

I resolved this by using filter-dkimsign (pkg_add opensmtpd-filter-dkimsign on openbsd)

[mneumann]

@mikejsavage exactly! Having X-Spam: yes headers in your outgoing email is ugly. And for some reason, any outgoing mail gets classified as Spam on my system (probably a bug/misconfiguration around rspamd). I will try to look at filter-dkimsign, thanks!

[poolpOrg]

I missed this PR somehow, can you please update the README.md to document the new option ?

[mneumann]

@poolpOrg Added a short comment in the README.md on how to use it. Please take a look if that makes sense.

[poolpOrg]

Perfect