Fix check for no credentials passed
@jonasdn
Submitted by Jonas Danielsson Link to original bug (#743392)
Description
So there is a race condition in the kernel. So that when we set SO_PASSCRED on an accepted socket it might not take effect before sending/reading.
One way to fix it would be to set SO_PASSCRED on the listening socket. Another way is to actually detect this in gunixcredentialmessage.c and fall back to PEERCRED.
We want to detect that credentials were not passed on a socket. But we cannot use '(uid_t) -1' as a condition since the kernel value is not the same size as uid_t. We need to explictly check for 65534.