Change the default 'rhizome.datastore_path' to "rhizome", ie, a sub-
directory of the instance directory.
Add a new erename() operation to "os.h".
Clean up the code around the fix 39f582cca (two commits ago) for a bug
introduced in 2014: the 'rhizome.db' file was always being created in
the instance directory, regardless of the setting of the
'rhizome.datastore_path' config option.
Whenever the Rhizome database is opened (by the server or by a CLI
invocation), if the database file does not exist but there is a
'rhizome.db' file in the legacy location, then move that file (and any
accompanying "blob" and "hash" sub-directories) to the configured
location.
Update copyright notices.
