Detect "empty" socket credentials on Linux

Linux uses struct ucred to pass over socket credentials. Historically
this has always worked in recievemsg, if SO_PASSCRED was set on the socket,
even if the remote side didn't pass any credits. But this change broke that:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=16e5726269611b71c930054ffe9b858c1cea88eb;hp=a9e9fd7182332d0cf5f3e601df3e71dd431b70d7

However, it doesn't actually fail getting the credentials, it just returns
an "empty" one, as initialized by cred_to_ucred() at:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=net/core/sock.c;h=b29ab61b029cf7f76fda992ecfcb8dcaa06b0483;#l756

So, we detect this and fail the credentials reading.

This actually happened in real life with gdbus acting as a server, as
gdbus expected an ucred but libdbus didn't send one.