@UNQREADB[[u,]filename,"key",handle[,length]]: Read a binary value from an existing key in an UnQlite database.
|u||Optional flag that the key is Unicode (UTF16)|
|filename||Database opened by @UNQOPEN|
|key||Key to read|
|handle||A binary handle returned by @BALLOC|
|length||Number of bytes to read. If not specified, read the entire binary buffer.|
Returns 0 if successful, or an error if not.