* avoid errors about undef values in strings from Perl strict mode
* only accept old hashes when password_hash_update is set to true

upgrade_sympa_password.pl.in:
* new command line options and associated POD documentation
* support for precaching hashes (useful for big sites using bcrypt)
* measure how long it takes to generate a hash (useful for bcrypt users)
* report on progress every 100 users processed when in verbose mode