Add SHA512_Final_Len() et al

An optimisation: programmers can now easily take just a part
of the digest without needing an intermediate buffer to hold
the entire digest
Cosmetic improvements to "str.h"
Refactor meshms seed string generation
Improve Rhizome AddUpdateSameVersion test

If 'rhizome add file' command does not return status 1,
then cat the standard error (log) to the test log
Fix assert_stdout_add_file() Rhizome test function

Correct 'author' and 'secret' fieldnames to '.author' and
'.secret'
Add strn_digest_passphrase()

Uses SHA512 internally
Accept '#passphrase' for bundle secret arguments

As well as a hex string.  Applies to CLI and also REST API.
Maximum pass phrase length is 80 ASCII chars.
Create authorless bundle from bundle secret

Until now, the bundle secret was never used to create a new bundle,
only to update a bundle whose secret was known from its creation.
Create authored bundle from bundle secret

Improve the logic in rhizome_fill_manifest() to handle the case where
the secret and author are both supplied, by setting the BK field so the
secret can be recovered from the manifest later