- Make `ipfs files rm --force /nonexistant` succeed when the path does not exist.
- Add shaness test for removing nonexistant paths
- Refactor duplicated code to find a parent dir into a function

I've been writing scripts against the files api, and having to stat things before removing them is a pain. So this PR aims to make --force do what I'd expect it to.

License: MIT
Signed-off-by: Oli Evans <oli@tableflip.io>