Check that a file is a DBF file.
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 Syntax

        GT_IsDbf(<cFileName>) --> lIsDbf

 Arguments:

      <cFileName> is the name of the DBF file. If the name has no
      extension it defaults to DBF.

 Returns:

      .T. if the file is a DBF file, .F. if not.

 Description:

      GT_IsDbf() is designed to check that a file is a Clipper DBF file.

 Examples:

      // Check if a file is a DBF file.

      ? GT_IsDbf("Life")

 Source: ISDBF.PRG

 Author:  Dave Pearson