Support `alert` as a secondary action in app-layer firewall policies.
To implement this a Signature object is created per policy that uses
alert, and this is stored in a hash table. When the policy is applied
the signature is looked up and used in the PacketAlert.
Ticket: #8566.
This module uses the sawp-pop3 crate to parse POP3 requests and responses
Features:
- eve logging
- events for parsable but non-RFC-compliant messages
Ticket: 3243