#64 cpu plugin: consider all cpu usage in linux

[José Romildo Malaquias]

Fixes to the cpu plugin:

* Recent versions of Linux has more fields in the cpu stats file that
the current cpu plugin takes into account: irq, softirq, steal and
guest. Added support for them.

* The struct cpu_stat now has only two fields: used and unused. They
measure cpu usage time since the last time it was checked.

* The original set of fields (u, n, s, i, w) became the local variables
user, nice, system, idle, iowait, irq, softirq, steal and guest in the
cpu_get_load_real for Linux.

* No need to declare the function cpu_destructor before its definition,
so it has been removed.

* Changed the value returned from cpu_get_load_real to indicate success
with true (1) and failure with false (0). It was returning success
with 0 and failure with -1.

* Renamed the paramater from s to cpu in the general (not linux, not
freebsd) version of cpu_get_load_real. That was a bug.

* The cpu load (used time divided by nonused time) changed from 1 to 0
when the total time is 0.

[José Romildo Malaquias]

cpu plugin: consider all cpu usage in linux, and other minor fixes