Add callbacks for displaying KSTAT_TYPE_RAW kstats

Add callbacks for displaying KSTAT_TYPE_RAW kstats

The current implementation for displaying kstats of type KSTAT_TYPE_RAW
is rather crude. This patch attempts to enhance this handling by
allowing a kstat user to register formatting callbacks which can
optionally be used.

The callbacks allow the user to implement functions for interpreting
their data and transposing it into a character buffer. This buffer,
containing a string representation of the raw data, is then be displayed
through the current /proc textual interface.

Additionally the kstats are made writable because it's now possible
to provide a useful handler via the existing ks_update() interface.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #296