Parametrization minimum password lenght

The goal about this patch is to parameterize the minimum password lenght on
users database and apply it on the general code.
The patch is applicable to the master branch.
We already tested it: it build and works as expected and nothing is found
broken,

Settings in postgresql.conf parametrization like following:

shared_preload_libraries = 'passwordcheck'
min_password_lenght = 12

example:

postgres=# create user prova with password 'eftghaki';
ERROR: password is too short
postgres=# create user prova with password 'eftghaki1234';
CREATE ROLE

In attach the file patch.

Best regards

Emanuele Musella & Maurizio Boriani