Bug 589: make random selection slightly more random. PHP's

Bug 589: make random selection slightly more random. PHP's
mt_rand() function has a maximum value of 2^31-1, which
is small enough that duplicate values can occur due to the
Birthday paradox, e.g. on the English Wikipedia. To fix
this, add a wfRandom() function that calls mt_rand()
twice to get the desired amount of randomness.