moduos.c

That one was missing in the module, even if it was available in the
vfs object. The change consist of adding the name and preparing the
call to the underlying vfs module, similar to what was already
implemented e.g. for remove.

Rename is useful by itself, or for instance for a safe file replace,
consisting of the sequence:

    write to a temp file
    delete the original file
    rename the temp file to the original file's name