main.cf template: silence postfix resource problems notifications. Refs #2824

* Increased default smtpd_proxy_timeout to 240 seconds
* Restricted notify_classes = software
nethserver-mail-common.spec: converted to plain .spec file. Refs #2803
postfix/master.cf: set syslog_name for different mail system components. Refs #2784
migration-import event: migrate SmartHost configuration. Refs #2804
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
main.cf: Customizable SMTP HELO value. Refs #2767

Set to empty string to get Postfix default.
network-create, delete events: force reload to read mynetworks table. Refs #2814
Mail spying / always Bcc. Refs #2750

* New "Always send a copy (Bcc)" field in UI
* Added main.cf/40always_bcc fragment
Imported Help files from mail-server. Refs #2750
Inline help: added "Always send a copy (Bcc)" field description. Refs #2750
amavisd.conf: read /etc/postfix/mynetworks.cidr. Refs #2814

Fixes trusted networks definition for amavisd.
main.cf: removed SMTP compliance checks. Refs #2768

This kind of restrictions are now enforced by mail-filter.
Merge branch 'b2750'

Refs #2843 #2841 #2814 #2804 #2784 #2768 #2767 #2750 #2723