Rename `ipfs files` commands to `ipfs mfs`

I'm not even sure we can do this without breaking too much stuff but I wanted to draw some attention to the fact that our family of `ipfs files` command, documented as

```
ipfs files - Interact with unixfs files.

Files is an API for manipulating IPFS objects as if they were a unix
filesystem.
```

actually seems to deal exclusively with MFS (not just *any* UnixFS file), that is (rewording this [question](https://github.com/ipfs/go-ipfs/issues/5858#issuecomment-448746950)), if I add content with `ipfs add` creating a UnixFS file, that file actually won't be accessible with the `ipfs files` command (e.g., `ipfs files ls`) since I need to add it to the MFS first. There doesn't seem to be any clear hint about this, not in the name of the command nor in its documentation.

Start by locating the CLI definitions and documentation for `ipfs files`, then compare their behavior with the proposed `ipfs mfs` name and the `ipfs add` and `ipfs files ls` examples. Check compatibility implications across the command family and document what must change; done means the naming and documentation consistently describe MFS without breaking existing users unexpectedly.

Written by the indexing model from the issue text.