macOS: Failed to mount /mntpoint: Resource busy (os error 16)

Hello,

I've just installed gcsf on macOS and unfortunately it wouldn't mount.

```
$ gcsf mount gd -s session
ERROR fuse::session > Failed to unmount /Users/badt/gd: Resource busy (os error 16)
$ sudo diskutil umount force gd
Unmount successful for gd
```

—is the procedure I have to go through to free up /Users/badt/gd after ^C out of gcsf.

This is definitely not an osxfuse issue, because sshfs works just fine here.

Start with the macOS mount and unmount path invoked by `gcsf mount gd -s session`, reproducing the failure by interrupting gcsf with Ctrl-C. Compare the reported `Resource busy` cleanup with the successful `diskutil umount force gd` result; done means interruption releases `/Users/badt/gd` without manual unmounting.

Written by the indexing model from the issue text.