OS X: Fix kernel connection closing on writes > 8128 bytes

This restores the mount_osxfusefs command from before the OS X mount
rewrite and sets iosize=4096. I have no idea why this option is
required, but it makes the connection not break.

This actually was the I/O error from #11, `git clone` this time died with
```
fatal: write error: Input/output error
fatal: index-pack failed
```
When this happens, `Conn.ReadRequest` returns an io.EOF error and Serve stops the loop.

Using `git bisect` I found dfc88d4ea5c77ec8b03401ca7404fe58abb8b9a1 was the first commit having this issue.

In case you can't reproduce the problem, I'll try to build a minimal filesystem showing the problem.