Add alloca_strndup() and strncpy_nul()
Support unrecognised manifest fields

Refactor Rhizome manifest parsing in preparation for command-line
field assignment
Refactor Rhizome manifest parsing

Now the parsing code uses the set/del functions for all supported
manifest fields, so no duplicate logic any more
Refactor manifest parsing

Consolidate tests for valid field names and values into
one place
"rhizome add file" manifest field=value varargs

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.
Fix bug in new RESTful MeshMS newsince logic

Introduced in a4be1b0, caused malformed JSON (missing comma)
Fix MeshMS failure caused by recent changes

Commit 65d6005 changed the code that creates a new manifest from
a bundle secret, and broke MeshMS