export iov_send_recv() and use it in iov_send() and iov_recv()

export iov_send_recv() and use it in iov_send() and iov_recv()

Rename do_sendv_recvv() to iov_send_recv(), change its last arg
(do_send) from int to bool, export it in iov.h, and made the two
callers of it (iov_send() and iov_recv()) to be trivial #defines
just adding 5th arg.

iov_send_recv() will be used later.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>