BtrvGetEqual goes to the previous record of the file based on the key
number that you pass and reads the record matching Value into DataVar.
The type of value passed has to match the type of data for which the key
is defined. If the key is a string then Value is a string. If BtrvResult
is zero, the data was found. If BtrvGetNext is called after this, the
next sequential record based on the key is read.
4 - Key Value not Found
6 - Invalid Key Number