Additional zevent infrastructure

The following changes extend the existing zevent infrastructure in the following two ways:

1) An EID (Error IDentifier) is added to every zevent.  It is a monotonically increasing value which can be used to problematically determine which events have been consumed and which haven't.  It is expected that the ZED (ZFS Event Daemon) will be the primary consumer.

2) The zpool_event_seek() function was added to allow seeking by EID for the zevent file descriptor.  This functionality was added to simplify the implementation of the ZED.