add files_darwin.go

Allows compiling and running on OSX.

- `hello` example not working: `mount_osxfusefs: failed to mount /Users/tzn/m@/dev/osxfuse0: Function not implemented;
- `unionfs` example mostly working
- `loopback` example mostly working

Most examples hang when terminated with ^C, will start consuming 100% CPU and will have to be killed.

ref. issue #16