variable "hosts" in on.config masked by code

When I use the variable "hosts" in on.config:

```
$ cat ~/.on.config
hosts="foo bar"
```

then that variable is not available for me afterwards:

```
$ on hosts list
$
```

and that's because that variable is being used by the "on" script internally by itself.

Idea: review code an replace all variables and functions with "on_$name" variants and document that as a "forbidden" namespace?
