socket-util: Clarify EAGAIN error code for make_unix_socket().

socket-util: Clarify EAGAIN error code for make_unix_socket().

make_unix_socket() can return EAGAIN in rare circumstances, e.g. when the
server's socket listen queue is full.  A lot of OVS callers interpret
EAGAIN as a "try again" error code, but in this case it means that the
attempt to create the socket failed.  So munge EAGAIN into another error
code to prevent that misinterpretation.