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