scrod/nv — pull request #312 by dchest (2012-08-03 21:21:33+00:00)
Title: Avoid modulo bias in password generator.

If the length of password characters alphabet is not a power of two,
the generated passwords are slightly non-uniform, that is, they
have modulo bias (see http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle#Modulo_bias)

Use arc4random_uniform(srclen) instead of arc4random() % srclen
to generate proper uniformly distributed characters.