Fix RRD Graphs

Replace the current date on graphs with graph begin and end date range.
Current time is not useful in some views.  Particularly Previous Period views.
	Add graph range variable to status_rrd_graph_img.php

Correct graph period ranges (start & end) in get_dates function.
	status_rrd_graph.php

Fix graph updates to use updated start and end times.
Rather than a static start time and "now" for end time.
Which:
# Causes Absolute Timespan, Current Period and Previous Period graph lengths to grow indefinitely .
# Makes Previous Period views turn in to previous period plus current period.
# Prevents the period of both Current and Previous period views from updating to the new period when a period time boundary is crossed.
    Move graph length array and get dates functions to status_rrd_graph.inc so they can also be reused by status_rrd_graph_update.php
	Create status_rrd_graph_update.php
	Update jQuery request to status_rrd_graph_update.php
	Move echo new lines and for loop close braces - status_rrd_graph.php
	Delete get_dates call

Consolidate if not previous period checks into a single check.
	Replace a bunch of these: if($offset != 0)
	With one of this: if($offset == 0) $end = now;

Set the 'average' granularity to be used for each graph length.
	status_rrd_graph_img.php
	Delete archives array and foreach loop
	Add switch statement for average variable

For 'Current Period' view, leave graph end variable set to period end rather than forcing it to current time (now).
	get_dates function
	Remove if($offset == 0) $end = $now;

Add Graph Update Interval Setting.
	status_rrd_graph.php
	status_rrd_graph_settings.php