opendkim: DKIM signing and verifying emails

opendkim: DKIM signing and verifying emails

The OpenDKIM package provides a service for signing and verifying
DomainKeys Identified Mail (DKIM) signatures. OpenDKIM consists of
a library that implements the DKIM service and a milter-based
filter application that can plug in to any milter-aware MTA, such
as Postfix or Sendmail, to provide that service to sufficiently
recent sendmail MTAs and other MTAs that support the milter
protocol.

This submission provides three new packages:
- libopendkim, a library for signing and verifying DKIM signatures,
- opendkim, the server application and the genkey script,
- opendkim-tools, a set of tools for configuring and testing OpenDKIM.

While at it, add PKG_BUILD_DEPENDS statement to sendmail's Makefile.
Travis CI buildbot sometimes fails to compile libopenssl before
starting to build sendmail. Since sendmail depends on libopenssl, the
whole Travis CI build process fails. Setting PKG_BUILD_DEPENDS
to "openssl", the directory name of libopenssl's Makefile, fixes the
problem.

Signed-off-by: Val Kulkov <val.kulkov@gmail.com>