Implementing perfect forward secrecy for i-MSCP

This will add perfect forward secrecy for several services:
- Apache: Recommendations taken from https://wiki.mozilla.org/Security/Server_Side_TLS. Configured better SSL defaults.
- Dovecot: Already active and used where possible. Logging format changed to show used encryption type
- Courier: Doesn't support it.
- Postfix: Needs pregenerated Diffie-Hellmann keys. Config file updated.
- MySQL: Not exposed to internet at the moment, so no action taken. This can be done later since MySQL fully supports it.
- ProFTPd: Already active and used where possible.


Important: On next update, the Diffie-Hellmann keys for postfix are generated. This process can take up to five minutes but is only done once.