Running a second instance of oguri replaces existing IPC socket

[vilhalmer]

This is a pretty uncommon thing to do, but it can happen for instance when testing a nested compositor. It could also happen by accident. Two options:

• When creating the socket, attempt to connect to it first to determine if an oguri already lives there. If so, disable the new instance's IPC. (Can't just check the file exists, could get stuck.)
• Add the pid to the filename, at the cost of the path no longer being predictable.

The second option could be improved by a configurable path.