Combine data-notify.sh with io-notify.sh

The data-notify.sh ZEDLET serves a very similar purpose to
io-notify.sh, namely, to generate a notification in response to a
particular error event. Initially, data-notify.sh was separated from
io-notify.sh since the "data" zevent does not (as I understand it)
pertain to a specific vdev device. This stands in contrast to the
"checksum" and "io" zevents (both handled by io-notify.sh) that can
be attributed to a specific vdev. At the time, it seemed simpler to
handle these two cases in separate scripts.

This commit adds support for the "data" zevent to io-notify.sh, and
symlinks io-notify.sh to data-notify.sh. It also adds the counts
for vdev_read_errors, vdev_write_errors, and vdev_cksum_errors to
the notification message.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>