tools/power turbostat: Add --list option to show available header names

It is handy to know the list of column header names,
so that they can be used with --add and --skip

The new --list option shows them:

sudo ./turbostat --list --hide sysfs
,Core,CPU,Avg_MHz,Busy%,Bzy_MHz,TSC_MHz,IRQ,SMI,CPU%c1,CPU%c3,CPU%c6,CPU%c7,CoreTmp,PkgTmp,GFX%rc6,GFXMHz,PkgWatt,CorWatt,GFXWatt

Signed-off-by: Len Brown <len.brown@intel.com>