BtrvGetGreaterOrEqu goes to the next larger or equal record of the file
based on the key number that you pass and reads the record greater 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