Unlock the receive buffer (of a RUM task) or change to a new
          receive buffer and receive actions.
            status.i2.v = NTRREQ( [reqadr.i2a.r],[maxlen.i2.r],[efn.i1.r],
            status.i2.v = NET_RECEIVE_REQUEST( [reqadr.i2a.r],[maxlen.i2.r],
         reqadr   the new receive buffer.  If defaulted, the address of
                  old receive buffer is retained as user must have
                  specified  a buffer to receive replies.  In either
                  case, the receive buffer is unlocked.
         maxlen   word containing the new receive buffer size in words.
                  If defaulted, then the size of the receive buffer
                  is unchanged (the old size is retained).
         efn      byte giving the number of an event flag to be set when
                  a request or USM reply is received.  If explicitly
                  defaulted, event flag 0 is used (on the VAX); implicit
                  defaulting (by a short argument list) leaves the event
                  event flag number unchanged.
         astadr   address of user's request AST routine.  If explicitly
                  defaulted, no user request AST will be invoked; implicit
                  defaulting leaves the request AST address unchanged.
         status   returns ACNET Global Status (sign-extended to 32-bits):
                      =0 for complete success
                      >0 for conditional success and pending
                      <0 for error condition
         The error codes returned are:
            -02 = local system out of space: no dynamic memory to
                  declare network AST, no free control blocks, or
                  no space in RUM task list.
            -21 = not connected to network
            -22 = insufficient number of arguments
            -23 = invalid message length: no buffer specified when
                  doing a RUM-connect (first call).
            -25 = illegal event flag
            -27 = name already in use by another RUM task
            -43 = system service error: no network process
            -44 = nontranslatable error: no XSC block
            -45 = internal network error
            -46 = network error #1: AST quota exceeded when trying
                                    to declare network AST
            -47 = network error #2: not owner of reply control block
            -48 = network error #3:
                - RUM Connect Block is already locked for modifications
                - control block vector already in use (only on 1st call)