Improve parsing of home-dirs in `ACTIVATE_PATH`.

All platforms should recognize `~` and POSIX platforms should also
recognize `~user`.

On Windows, the home path should be deduced properly from `HOMESHARE` or
`HOMEDRIVE` in combination with `HOMEPATH`.  As I don't have an
environment where `HOMESHARE` is relevant, nor one where "home" and
`USERPROFILE` are different, YMMV.

Closes #1.