Fixed fdisk.sh to be compatible with Debian (in addition to CentOS).

* fdisk(1) on CentOS vs. Debian are start off with opposite defaults for DOS
  compatibility and cylinder vs. sector display, which was fixed by converting
  commands (which toggle) to command-line flags (which set).
* removed now-stale comment about 500GB disks and ratio of 50 since these
  scripts handle arbitrary-sized disks
* fixed the command to compute the amount of free disk space on Debian
  correctly; documented the differences in the mounted filesystems on CentOS vs.
  Debian