@UNQKVFA[[u,]filename,"key",filename,length] : Append the contents of a file 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|
|filename||File to append to the existing value|
|length||Length (in bytes) to write (or -1 for the entire file)|
The maximum file size is dependent on the amount of RAM and disk space available.
Returns 0 if successful, or the error text if not.