The existing code errors as EINVAL any attempt to set a value for
SO_PEERCRED via setsockopt() on an AF_UNIX/AF_LOCAL socket. But to
enable the workaround set_no_getpeereid behavior for Python one has
to be able to set SO_PEERCRED to zero. Ergo, this patch. Python has
no way to specify a NULL pointer for 'optval'.
This v2 of patch allows the original working (i.e., allow NULL,0 for
optval,optlen to mean turn off SO_PEERCRED) in addition to the new
working described above. The sense of the 'if' stmt is reversed for
readability.