cli: top.py CLI replacement

Shows thread information from the API. It is advised to use this command
outside of an interactive shell since there's no proper way to stop it, yet,
except using ^C.

For example:
```
export OSV_API=http://192.168.122.89:8000
./cli -- top -s # :)
```

Closes #489.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vfs: fix comment describing locking in get_arcbuf

vn_lock is no longer taken when calling VOP_CACHE,
thus the locking description should be fixed.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>