Fix test framework: bug in assertStdoutIs() et al

Could overwrite the latest stdout or stderr file produced by execute()
Fix prototype of bcopy()

Add const to void * src argument
Rewrite bundle author authentication

Replaced 'int has_author' manifest element with new 'enum authorship'
element to record the result of author authentication, to avoid
repeating expensive crypto operations.

Separated the handling of bundle secret arguments from author lookup and
authentication.  The new rhizome_apply_bundle_secret(m,bsk) is now
called at the top level to set the manifest secret key (if it
validates), and thereafter there is no need to pass the 'bsk' argument
to any other functions, as they can simply check the 'haveSecret' field
of the manifest.

Removed rhizome_extract_privatekey() which combined author lookup and
bundle secret validation, and replaced it with functions that only deal
with the author: rhizome_lookup_author() and rhizome_authenticate_author().

Renamed other functions to make their purpose and effect clearer.

Formalised the semantics of only storing AUTHENTICATED author SIDs in
the 'author' column of the MANIFESTS table, which necessitated a change
to a 'rhizomeops' test case: when adding a file using a BK-less
manifest, the author column is set to null, so the Rhizome list output
does not show the bundle as ".fromhere" and does not give an author for
that bundle.