Generate zevents for speculative and soft errors

Generate zevents for speculative and soft errors

By default the Solaris code does not log speculative or soft io errors
in either 'zpool status' or post an event.  Under Linux we don't want
to change the expected behavior of 'zpool status' so these io errors
are still suppressed there.

However, since we do need to know about these events for Linux FMA and
the 'zpool events' interface is new we do post the events.  With the
addition of the zio_flags field the posted events now contain enough
information that a user space consumer can identify and discard these
events if it sees fit.