In arc_summary, replace getopt with argparse.

This script, zfs/cmd/arc_summary/arc_summary.py, originally used getopt
to parse command line arguments. This presents a problem, as getopt is
not user friendly, doesn't provide built-in help or descriptive error
messages, and can be difficult to maintain. Furthermore, the original
code relied on a try-catch block in lieu of an if statement in order to
determine whether to display all output pages or only specific ones.

This commit fixes those issues by using the python library argparse.
Argparse supports built in help and error messages while simplifying the
code required to handle the presence or absence of flags. This will be
important in a later commits which will implement additional flags.
Created new output page to report spl stats

Added two new functions to track spl stats, get_splstats() which reads
the spl stats file to extract statistics from it, and
_spl_summary(Kstat) which reports the statistics.
Merge branch 'arcsummary' of https://github.com/p4tino/zfs into arcsummary

Conflicts:
	cmd/arc_summary/arc_summary.py