Move create_rhizome_identities() from testdefs_rhizome.sh to
create_identities() in testdefs.sh
Rename create_identity() in testdefs.sh to create_single_identity(), uses
create_identities() to do its work.
Handle manifests with empty payloads:
- assert_manifest_complete() asserts for a filehash field if the
filesize is non zero, otherwise asserts for NO filehash field
- assert_stdout_add_file() ditto
- unpack_manifest_for_grep() allows an empty name if payload also empty
create_rhizome_identities() sets SID and SID{I} vars
assert_rhizome_list() dumps stderr if any assert fails