Log Message:
Add libdkim: Portable library that signs and verifies emails

libdkim is a portable library that signs and verifies emails using the
OpenSSL library.

DomainKeys Identified Mail (DKIM) defines a domain-level authentication
framework for email using public-key cryptography and key server
technology to permit verification of the source and contents of messages
by either Mail Transport Agents (MTAs) or Mail User Agents (MUAs).