Support more platforms for `user_dir` and `user_dirs`
Support more platforms for `user_dir` and `user_dirs`

Prior to this change, only the Linux and Redox OS targets would compile
an implementation.

This change broadens the list of targets which will conditionally
compile the `user_dir` and `user_dirs` functions such that all Unix
family targets (except macOS and iOS) can use this implementation.

dirs-dev/dirs-sys-rs