- sendmsg: the accumulation of multiple in-flight file descriptors was
already described in the comments; now the code actually does what
the comments say :) -- also, added robustness in case of a failure;
- recvmsg: only create a socket rights message if there are file
descriptors pending at all;
- recvmsg: copy back the control message length;
- recvmsg: use CMSG_SPACE instead of CMSG_LEN to compute sizes.
Not sure if all of this is now working according to specification,
but at least tmux seems to be happy with it.
Change-Id: I8d076c14c3ff3220b7fea730e0f08f4b4254ede5
9e3e032c26