give password plugin drivers more power

Allow password drivers more control over the entire password changeing
processes not just the save action. Allow them to perform old/new
password comparisons and also password strength checking.

*) allow password drivers override default password comparisons (eg

new is not same as current) #6473

*) allow password drivers override default strength checks (eg allow

for 'not the same as last x passwords')

*) allow separate password saving and strength drivers for use of

strength checking services eg HaveIBeenPwned.com #5040

*) allow drivers to define password strength rules displayed to the

user

*) rename password_require_nonalpha config option to

password_check_strength to reflect new strength checking
possibilities (added backwards compatibility)