socket: fix bug #65260 (SCM_RIGHTS)

The data for messages of type SOL_SOCKET/SCM_RIGHTS was not being
passed correctly. There were actually two bugs: (1) the number of file
descriptors being passed was being read incorrectly (the length of the
cmsg array was being read instead of that of its 'data' element), as a
result it was generally being reported as always three elements
('level', 'type' and 'data') and (2) the allocated block for writing
the file descriptors was being acessed incorrectly because a 1-based
counter was being used as if it was 0-based.

Any of these two bugs would probably be enough to cause heap
corruption.
socket: support unix paths in the abstract namespace

Those starting with '\0'.
NEWS for 710150c and e2744f1