more aggressively sanitize dbus paths
paths may only contain alpha numeric ascii characters + _ + /. Also
assert the path is well-formed while at it.
```
The path may be of any length.
The path must begin with an ASCII '/' (integer 47) character, and
must consist of elements separated by slash characters.
Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_"
No element may be the empty string.
Multiple '/' characters cannot occur in sequence.
A trailing '/' character is not allowed unless the path is the root
path (a single '/' character).
```