check_disk: option to ignore root-reserved space

Hello,

I didn't see how to ignore root-reserved space in check_disk so here is a patch adding an runtime option for this.

I took the letter 'f' (freespace-ignore-reserved) but maybe it's not the most intuitive. Feel free to change.

Extract from commit message :
> Before / after:
> 
>     ./check_disk -w 2% -c 1% /home
>     ./check_disk -w 2% -c 1% -f /home
>     DISK CRITICAL - free space: /home 272 MB (1% inode=83%);| /home=17551MB;18401;18589;0;18777
>     DISK CRITICAL - free space: /home 272 MB (1% inode=83%);| /home=17551MB;17466;17644;0;17823
> 
> Well, it doesn't really give me more space :) but the perfdata are a
> little bit more coherent : free + used = total.