Implement fallback if wipefs is not available or fails (#688882)

RHEL/CentOS 5.x uses util-linux 2.13 which predates the wipefs command.
Also the wipefs command may fail.  Therefore implement a fallback which
caters for both these cases by overwriting all possible file system
super blocks with zeros at the start of the partition.

The "MUST be cleared" requirement stated in the first patch is now
satisfied.  Therefore the status of the "clear old file system
signatures" step, as returned by erase_filesystem_signatures(), now
reflects the success of the wipefs command or the internal write zeros.
Failure of both will stop all operations, matching what happens with all
other steps.

Bug #688882 - Improve clearing of file system signatures