Greatly improve quality of automatically generated passwords

Greatly improve quality of automatically generated passwords

Previously the amount of characters for each type was deterministic, as
the character types were chosen in a round robin fashion, instead of
randomly choosing from the entire character set. This lead to about
47 bit of entropy with the default length of 8 characters. Additionally
str_shuffle does not use a SRNG, which may have reduced the entropy even
further.

The new algorithm, choosing fairly from the whole range of alphanumeric
characters with a default length of 12 characters provides about 71 bits
of entropy.