[MLton] basis library bug

[Henry Cejtin]

The  basis  library  spec  for the Socket structure clearly has a typo in the
types  of  recvVecFrom,  recvVecFromNB,  recvVecFrom'   and   recvVecFromNB'.
Consider  the  corresponding  recvArrFrom,  recvArrFromNB,  recvArrFrom'  and
recvArrFromNB'.  The latter correctly require that the address family of  the
returned sock_addr match that of the argument socket.  The former allow it to
be any sock_addr type at all.  Clearly it is not up to  the  caller  to  pick
what kind of address things came from.