detect: set ACTION_ALERT for rules that should alert

detect: set ACTION_ALERT for rules that should alert

Replaces default "alert" logic and removed SIG_FLAG_NOALERT.

Instead, "noalert" unsets ACTION_ALERT. Same for flowbits:noalert and
friends.

In signature ordering rules w/o action are sorted as if they have 'alert',
which is the same behavior as before, but now implemented explicitly.

Ticket: #5466.