Add functionality to report SPL stats.

Added two functions, one to gather spl stats and one to print them out.
Fix bug in arc_summary.py, -p option now works.

The -p option is designed to specify which page of output to display. It
wasn't working because the function that gathers statistics, get_Kstat
was only being called in the 'call_all' function which was never called
if the -p option was in place. Owing to this, an exception would be
raised in the option handling code in the main function, which would
then call 'call_all', causing all pages of output to display,
effectively negating the point of the -p option.

This fix ensures that get_Kstat is called in the main function, thus
enabling a single page of output to be displayed without running into an
exception.
Refactor arc_summary to simplify -p processing

The -p option is used to specify a specific page of output to be
displayed. If omitted, all output pages would be displayed.

arc_summary, as it stood, had really kludgy processing code for
processing the -p option. It relied on a try-except block which was treated
as an if statement and in normal operation would fail any time a user
didn't specify the -p option on the command line. When the exception was
thrown, the script would then display all output pages. This happened
whether the -p option was omitted or malformed. Thus, in the principle
use case, an exception would be raised in order to run the script as
desired. The same except code would be called regardless of the exception,
however, and malformed -p arguments would also cause the script to
execute.

Additionally, this required the function which handles the case where
all output pages were to be displayed, _call_all, to be potentially called from
several locations within main.

This commit refactors the option processing code to simplify it and make
it easier to catch runtime errors in the script. This is done by
specializing the try-except block to only have an exception when the -p
argument is malformed. When the -p option is correctly selected by the user,
it calls a new function called _call_one, which will only display one
page of output. Likewise, _call_all now relies on _call_one for
output processing, making it potentially easier in the future to catch
runtime exceptions as there is only one point of failure. Finally, _call_all
is only called from one location within main.
Remove some commented out code.
Add error checking for missing arcstat variables.

Added try-except block around unSub function calls.