main.cf: smtpd_relay_restrictions set to empty value. Refs #2814
main.cf: enforce submission_whitelist on client, helo and sender restrictions. Refs #2814
main.cf: new postfix/AccessPolicies prop. Refs #2814

The AccessPolicies prop is a comma separated list of policy
identifiers. An access policy is expected to change the access
restrictions enforced by smtpd and/or submission processes.

An access policy implementation is a function added to the
%access_policies global hash in main.cf template.

Functions are invoked by 10access_policy fragment. See
01access_policy_smtpauth for instance.

Moreover this change:

- Obsoletes postfix/SubmissionPolicyType prop applying a migration
  fragment.

- Imported policies from nethserver-mail-server into
  01access_policy_*.

- Activates smtpauth policy on migration-import event
mynetworks template: grant access from trusted networks if explicitly requested. Refs #2814
Template "submission_whitelist" obsoleted by "access" table. Refs #2814

The old submission_whitelist template-custom fragments are still
included into access, to retain backward compatibility.
Changed mynetworks table format to cidr: Refs #2814
main.cf: updated distro defaults for Postfix package. Refs #2750