Get the number of free clusters on a drive.
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 Syntax

        GT_ClusFre([<ncDrive>]) --> nClusters

 Arguments:

      <ncDrive> is an optional parameter that is the id of the drive
      to be read. This paramater can be either a character value who's
      first character is taken as the drive letter or a numeric value
      where 0 = Default, 1 = A:, 2 = B:, etc... If no parameter is
      passed the default drive is used.

 Returns:

      The numer of free clusters on the drive. If the drive is invalid
      the return value will be -1.

 Description:

      GT_ClusFre() can be used to find the number of free clusters on
      a disk.

 Examples:

      // Each of the following print the number of free clusters found
      // on drive C:

      ? GT_ClusFre("C:")
      ? GT_ClusFre("C")
      ? GT_ClusFre("Clipper")         // First letter is used only.
      ? GT_ClusFre(3)

      // The next two print the number of free clusters in the current
      // drive.

      ? GT_ClusFre(0)
      ? GT_ClusFre()

 Source: CLUSTFRE.C

 Author:  Dave Pearson