Use PHP 5.5+ new password functions, closes #2182

Warnings:

• $core->auth->crypt($pwd) doesn't return twice the same result for a single $pwd, so if you need this old behaviour use the $core->auth->cryptLegacy($pwd) instead.
• $core->auth->checkPassword($pwd) must be used with an uncrypted password string as argument.
• if you need a unique UID/key, use http::browserUID(DC_MASTER_KEY.$core->auth->userID().$core->auth->cryptLegacy($core->auth->userID())). (may be refined in future)