mail-filter/opendkim: use /var/lib/opendkim in pkg_config.

mail-filter/opendkim: use /var/lib/opendkim in pkg_config.

The keys that are generated by opendkim-genkey are data, in a sense,
and not configuration files. As a result, I think it's more appropriate
to store them in /var/lib/opendkim than in /etc/opendkim where they were
previously stored. This commit moves the keys, and also tightens the
permissions on them a bit so that the "opendkim" user can only read them.

Signed-off-by: Michael Orlitzky <mjo@gentoo.org>
Package-Manager: Portage-2.3.62, Repoman-2.3.11