add DKIM options to PMG::Config

add DKIM options to PMG::Config

This patch adds a new registered file to PMG::Config: /etc/pmg/dkim/domains.
* It holds a list of domains for which mail will be signed if received on the
  internal port (26).
* The domain of a mail is determined by it's envelope-sender
  (not the From: header)
* If the file does not exist mail is signed for the relay domains as a sensible
  fallback

Additionally it adds 3 new options to the admin section of pmg.conf:
* dkim_sign - whether DKIM signing should be attempted
* dkim_sign_all_mail - wheter the outbound mail should be signed irrespective
  of sender-domain
* dkim_selector - the selector used for signing (a label of the DKIM TXT record
  and present in the signature - thus linking the signature to the used key
  - see RFC6376)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>