users: Use libpwquality for password handling

Using a library for password generation and quality checking
has the obvious benefit that we can have centralized policy
for password quality, and our homegrown code for this was not
a beauty in the first place.

This commit drops the use of /usr/bin/agp for generating
random passwords.

https://bugzilla.gnome.org/show_bug.cgi?id=676396