EntityControlFile.update_checksums now handles absent files

update_checksums() crashed if (ddr-local) Entity.files listed
a file that was not present in the filesystem.
For example, a git-annex file that had not been git-annex-gotten.
With this change the function inserts "UNKNONWSIZE" for files
where the filesize cannot be determined.
entity_annex_add now takes optional entity arg for checksumming

This allows ddr-local to pass in DDRLocalEntity objects which
have an overridden checksums() method that correctly deals with
files that are not present in the filesystem.  For example,
symlinks to absent git-annex files (i.e. important).