Represent the home directory as ${HOME} or %UserProfile%

We'd like to avoid including home directory paths in our log messages, since they are frequently things like C:\Users\Jane Doe\ or '/home/YourNameHere/'.

This branch adds functionality to fs-mistrust¹ to replace most variants of our home directory path with ${HOME} (or `%UserProfile% on Windows), and then uses that functionality to redact filenames in our log and error messages.

Thanks to @sready for telling me about %UserProfile%.

Closes #555 (closed).

---

1. I put it in fs-mistrust to avoid a dependency on e.g. tor-basic-utils, and because fs-mistrust can sometimes reach its own notion of canonical paths (see !699 (merged)). ↩