[Dovecot] Enable mail logger plugin

[Alexandre Ellert]

> @Alexandre: You've removed the "$mail_plugins" in the IMAP section as Peter already pointed out?

I'm not sure to fully understand how mail_plugins directive works.

Please correct, if I'm wrong :

Global mail_plugins lists common plugin used by all protocol :
> mail_plugins = quota mail_log notify
Later, within each protocol declaration, I need to reference $mail_plugins variable (i.e. mail_plugins = $mail_plugins …)

Each protocol can use its own mail_plugins with an override :
> protocol imap {
>  mail_max_userip_connections = 80
>  mail_plugins = quota imap_quota antispam
> }
In that case mail_log notify are not enabled for protocol imap.

So, I have two options :

1. List all mail_plugins explicitly for protocol imap, like this :
mail_plugins = quota mail_log notify
protocol imap {
 mail_max_userip_connections = 80
 mail_plugins = quota imap_quota antispam mail_log notify
}

2. Use $mail_plugins variable to include global mail_plugins into protocol imap :
mail_plugins = quota mail_log notify
protocol imap {
 mail_max_userip_connections = 80
 mail_plugins = $mail_plugins imap_quota antispam
}

Alexandre