core: Make brute-force akills optional (per user)

[siniStar7]

** Suggested by Maro on IRC: **

Allow users to optionally opt-in or out of the authentication brute-force protection. (An account with 10 failed IDENTIFICATION attempts results in AKILLs for any further failed identification at 10+ times)

This could be made to be an opt-in or opt-out feature, set by each individual user. It would probably be best to have this be an OPT-IN feature or setting as opposed to being automatically set with no way of unsetting.