Sets options on a socket.
    The $QIO equivalent is the IO$_SETMODE function.
    Format
      #include  <types.h>
      #include  <socket.h>
      int setsockopt  ( int s, int level, int optname, char *optval,
                      int optlen ); (_DECC_V4_SOURCE)
      int setsockopt  ( int s, int level, int optname, const void
                      *optval, size_t optlen ); (not_DECC_V4_SOURCE)