Distinguish between fatal and user-supplied-input errors, return
error descriptive text in a strbuf to allow dynamic content, apply
user-supplied field assgnments/deletions _after_ copying existing
manifest fields, use exit status 4 (invalid manifest) when applying
journal append to a non-journal or vice versa
Put manifest creation logic into new rhizome_bundle_add_file()
function, in preparation for implementing new HTTP POST
/restful/rhizome/append request
Several 'rhizomeops' tests fail
Now a new manifest can be formed, and an existing manifest modified, by
giving "field=value" or "!field" command-line arguments at the end of
the "rhizome add file" command (after the <bsk> argument).
This makes it easier to script rhizome operations.
