unix domain socket not removed if it exists

When using a unix domain socket, config:
"API": [ "/ip4/127.0.0.1/tcp/5001", "/unix/var/db/ipfs-go/socket/api.sock" ],

the first time it starts up, it works fine, but the second time it fails:
```
Error: serveHTTPApi: manet.Listen(/unix/var/db/ipfs-go/socket/api.sock) failed: listen unix /var/db/ipfs-go/socket/api.sock: bind: address already in use
```

It is common for programs to either use a lock file, or just simply remove it on start.

I've worked around the issue by removing it in my startup file, but a better solution that is more generic would be nice.