BtrvGetLessOrEqu goes to the next smaller or equal record of the file
based on the key number that you pass and reads the record less than or
equal to 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
BtrvGetPrev is called after this, the next sequential record based on
the key is read.
6 - Invalid Key Number