[4.1] [BUG FIX] [IMPROVEMENT] Improved Random

I've added a `fullRandom` function which will attempt to generate a truly random string.

I have updated the current `random` function to use this new secure function if available, and then to fallback on `quickRandom`.

I've fixed the errors introduced by @taylorotwell in laravel 4.1.28 whenever the `validMac` function was called causing: `You need to specify a file path to store the seed`.

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Related to #4130. Fixes #4182 and laravel/laravel#2849.