@UNQKVBA[[u,]filename,"key",bhandle,length] : Append a binary blob to the value of an existing UnQlite key/value pair.
|u||Optional flag that the key and value are Unicode (UTF16)|
|filename||Database opened with @UNQOPEN|
|key||Key to update|
|bhandle||Binary handle returned by @BALLOC|
|length||Length of binary buffer to write (or -1 for the entire buffer)|
Returns 0 if successful, or the error text if not.