Add `force` option for replacing existing files

[lheckemann]

I wrote this in hopes that I could manage /etc/shadow with impermanence. Unfortunately, this does not play along with the rename-based atomic update mechanism applied by both passwd and nixos's update-users-groups.pl, so I don't have a good example to use in the documentation...

[jedesroches]

This would definitely be cool! An example application is tracking the /var/lib/systemd/timesync/clock file. Tracking the directory is a workaround, but I like being specific and this would help :)

[KiaraGrouwstra]

as others linked here might wonder as well, what might be alternatives for shadow (/ passwd)? perhaps as a bonus question: is there a way one might check if such a particular file should work with this, or fall into a similar category as those exceptions?