If the control message of IP_SENDSRCADDR did not fit into the socket

If the control message of IP_SENDSRCADDR did not fit into the socket
buffer together with an UDP packet, sosend(9) returned EWOULDBLOCK.
As it is an persistent problem, EMSGSIZE is the correct error code.
Split the AF_UNIX case into a separate condition and do not change
its logic.  For atomic protocols, check that both data and control
message length fit into the socket buffer.
original bug report from Alexander Markert
discussed with jca@; OK vgross@