leds: ledtrig-activity: Add a system activity LED trigger

The "activity" trigger was inspired by the heartbeat one, but aims at
providing instant indication of the immediate CPU usage. Under idle
condition, it flashes 10ms every second. At 100% usage, it flashes
90ms every 100ms. The blinking frequency increases from 1 to 10 Hz
until either the load is high enough to saturate one CPU core or 50%
load is reached on a single-core system. Then past this point only the
duty cycle increases from 10 to 90%.

This results in a very visible activity reporting allowing one to
immediately tell whether a machine is under load or not, making it
quite suitable to be used in clusters.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>