automatic re-hash of the password when the hash method is deprecated

[laurentj]

When importing user data into the database, the hash method used into the previous application may not be the current hash method.

So we need to implement the change of the hash during the authentication, and so we need two parameters:

• a flag to enable the feature
• the name of the function that hash a password with the old hash method, to allow to check if the given password is ok