Implement --ignore for file properties

(cherry picked from commit 6c0f1410e11f76577d22433affbb32e0582c81f2)

Conflicts:
	python/make_differential_dfxml.py
Add sanity check for vetting counting strategies
Remove logic for reading pre-diff DFXML files

Retaining objects now creates the same tallies, with these exceptions:

* These counts are inflated by adding "Ghost" FileObjects (deleted
  files that have original object appended):
    - Total file object tally of latter image
    - Null/null name allocation status of latter image
    - Null/null inode allocation status of latter image
* One additional inode is counted - the null partition, null inode
  added from all "Ghost" objects.

Signed-off-by: Alex Nelson <ajnelson@cs.ucsc.edu>
Add unallocated, unmatched counter
Split difference mask and ignore sets