libnbd: add ENOTSUP errno, add LIBNBD_CMD_FLAG_FAST_ZERO to nbd_zero() and nbd_aio_zero()
ENOTSUP
LIBNBD_CMD_FLAG_FAST_ZERO
nbd_zero()
nbd_aio_zero()
nbdkit: add support in the following filters and plugins XXX worth flagging which additions are always fast, always slow, conditional?