Want some way to mount kbfs as nobrowse

Having the `/keybase` volume show up on my desktop (OS X) is irritating. I'd prefer to have it mounted with the `nobrowse` option.

I tried running `mount -u -o nobrowse /keybase`, but all that did was show me the output

```
KBFUSE mount version 3.1.0

This program is not meant to be called directly. The KBFUSE library calls it.

Available mount options:
...
```

Start by tracing how the OS X /keybase volume is mounted and where KBFUSE mount options are assembled; the issue shows that invoking mount directly only prints the available options. Determine where a nobrowse option can be passed during the normal mount flow, then verify that /keybase no longer appears on the desktop while remaining mounted.

Written by the indexing model from the issue text.