Use statvfs on NetBSD.

One field name in struct statvfs is different than one Solaris; adapt.
Fixes build on NetBSD, since statfs support was removed.
just eyeballing this; it looks longer than 80 columns.

To keep the length down, can you switch it to this:

```
// NetBSD defines udata as intptr type, so the cast is necessary
struct watchman_file *file = (void *)root->keventbuf[i].udata;
```

I think this is more honestly expressing what we want anyway: on FreeBSD and also on OS X (even though we no longer use kqueue there) `udata` is defined as a void pointer.