os/alert: Added DB serialization.

Moved data structures into os_alert.h.

Refactored os_cmd_alert_add into parse_alert so that it can be used for
deserialize.
os/alert: Added USER criteria.

The nick and user criteria both use the same underlying type:
alert_pattern_criteria_t, but with different constructors.
os/alert: The NOTICE action now shows the alert id.
os/alert: Add GECOS criteria.

pattern_display now handles multi-word patterns.
os/alert: add HOST criteria.