New plugin: turbostat (CPU frequency and C-state residency)

This plugin is based on https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/tools/power/x86/turbostat/turbostat.c

It reads CPU frequency and C-state residency on modern Intel turbo-capable processors.

Here are examples of produced data (using CGP, see https://github.com/Feandil/collectd-turbostat/tree/master/CGP-plugin for the CGP plugin).
Example 1 (Small load):
![collectd-turbostat_cgp](https://cloud.githubusercontent.com/assets/1488247/3334380/fb54fb0a-f7fe-11e3-9f4c-67b501030705.png)
Example 2 (Big compilation near the end):
![collectd-turbostat_cgp-2](https://cloud.githubusercontent.com/assets/1488247/3334381/039a95d6-f7ff-11e3-818d-3684ae7cabc1.png)





Just tried to compile it without _GNU_SOURCE:
> turbostat.c: In function 'cpu_is_not_present':
> turbostat.c:219:2: warning: implicit declaration of function 'CPU_ISSET_S' [-Wimplicit-function-declaration]
> turbostat.c: In function 'cpu_migrate':
> turbostat.c:260:2: warning: implicit declaration of function 'CPU_ZERO_S' [-Wimplicit-function-declaration]
> turbostat.c:261:2: warning: implicit declaration of function 'CPU_SET_S' [-Wimplicit-function-declaration]
> turbostat.c:262:2: warning: implicit declaration of function 'sched_setaffinity' [-Wimplicit-function-declaration]
> turbostat.c:549:2: warning: implicit declaration of function 'CPU_FREE' [-Wimplicit-function-declaration]
> turbostat.c: In function 'topology_probe':
> turbostat.c:1273:2: warning: implicit declaration of function 'CPU_ALLOC' [-Wimplicit-function-declaration]
> turbostat.c:1273:18: warning: assignment makes pointer from integer without a cast [enabled by default]
> turbostat.c:1279:2: warning: implicit declaration of function 'CPU_ALLOC_SIZE' [-Wimplicit-function-declaration]
> turbostat.c:1290:19: warning: assignment makes pointer from integer without a cast [enabled by default]

These are declared in /usr/include/sched.h and are indeed protected by a `#ifdef __USE_GNU`