Receives bytes from a connected socket and places them into a
    user-provided buffer.
    The $QIO equivalent is the IO$_READVBLK function.
    Format
      #include  <types.h>
      #include  <socket.h>
      int recv  ( int s, char *buf, int len, int flags );
                (_DECC_V4_SOURCE)
      size_t recv  ( int s, void *buf, ssize_t len, int flags );
                   (not_DECC_V4_SOURCE)