Remove chntpw and reglookup in favor of hivex*

Use hivexget and hivexregedit instead of reglookup and chntpw to browse
and alter the windows registry. The hivex utilities are better designed
and maintained
helper: add missing program checks in ./configure

Add checks for tune2fs, sfdisk, sgdisk and eatmydata
docs: Add a configuration section

Add a section where it is explained how snf-image can be configured
via the /etc/default/snf-image file.