exchangers: Use inheritance in xt_exchanger_irecv_send.

src/Makefile.am

src/xt_exchanger.c

src/xt_exchanger_internal.h

src/xt_exchanger_irecv_send.c

src/xt_exchanger_irecv_send_internal.h

src/xt_exchanger_vtable.c

src/xt_init.c