nixos/users-groups: dump values of password options if multiple options have definitions

This was suggested since it might make it a little easier to identify
the places where the definitions come from.

Retrieving the effective definitions from the module-system seems
non-trivial, especially for submodules though, hence only the values are
shown for now.

I'd argue that especially the `password` option are mostly a convenience
thing for test setups. If the password is an actual secret, it should be
treated as such, i.e. `hashedPasswordFile` should be used.

For the `shadow` VM test, the new section of the warning looks like
this:

    The values of these options are:
    * users.users."leo".hashedPassword: "$6$ymzs8WINZ5wGwQcV$VC2S0cQiX8NVukOLymysTPn4v1zJoJp3NGyhnqyv/dAf4NWZsBWYveQcj6gEJr4ZUjRBRjM0Pj1L8TCQ8hUUp0"
    * users.users."leo".hashedPasswordFile: null
    * users.users."leo".password: null
    * users.users."leo".initialHashedPassword: "!"
    * users.users."leo".initialPassword: null

0 files changed, 0 insertions, 0 deletions