This allows us to use UNIX filesystem permissions for access control.
Previously, any process in the network namespace could connect to it,
which is insecure for obvious reasons.
Verified that correct permissions are set:
```
# ls -lisa /run/bridge/
total 8
31996269 4 drwxrwxrwx 2 root root 4096 Nov 23 21:58 .
14676759 4 drwxr-xr-x 1 root root 4096 Nov 23 21:58 ..
31996306 0 srwx------ 1 root root 0 Nov 23 21:58 agent.sock
```
Fixes#119
8306a83833