These next functions are to centralize all accesses to one or more
 binary files in Clipper.

    These functions check all kinds of errors that could be produced
 in a Clipper program when it accesses binary files, like, the file
 doesn't exist, or trying to access a file without locking it.

    Besides, you'll be able to forget the I/O errors because all file access
 functions retry the operation and inform the user if there's some error.