check_disk: option to ignore reserved space

This commit add a runtime option to ignore the root-reserved space
when calculating perfdata.

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.

Note that there are still a rounding issue on the percent calculation
(cf calculate_percent() comments, taken from coreutils 'df' command).
272/17823 -> 1.526%

Since the warning/critical/min/max data are usually not saved in RRD DB,
you can activate the option without seeing landslides in the graphs.
Merge branch 'master' of https://github.com/bugness-chl/nagios-plugins

* 'master' of https://github.com/bugness-chl/nagios-plugins:
  check_disk: option to ignore reserved space

Conflicts:
	plugins/check_disk.c
NEWS: Mention new "check_disk -f" option

Closes #22.