Dkim sign: fix crash issue #560

* initialize headers_to_sign as empty array, because that's what everything after will try accessting it as
* shed camelCase
* whitespace style updates
* consistent use of ; chars (syntastic)
* extract signing domain from From header with fallback to legacy use of env MAIL FROM domain. (In most cases, the header From domain is the same as the envelope MAIL FROM domain. Now that address-rfc2822 is a dependency, attempt to get header From domain as that's what DMARC alignment tests check.)
* moved config parsing into register with cb