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Rely on link instead of FoBiS recursion

@victorsndvg discovered a subtle bug. From his report:

I think there is a bug (in release mode) using FLAP while parsing CLI multiple times.
If I run the code below, the second time that I call `parse(error)`, `error` value is not initialized from the `parse()` procedure.
```fortran
     type(Command_Line_Interface)  :: cli
     integer :: error
     cli%parse(error)
     ...
     cli%free()
     ...
     cli%parse(error)
     ...
     cli%free()
```

Why:

The bug do not free the CLI properly and do not return the correct (null) code the other times it is parsed.

This change addresses the need by:

@victorsndvg provided a perfect patch:

First, force a successful `error` code at the beginning of the `parse()` procedure in this line: https://github.com/szaghi/FLAP/blob/master/src/lib/flap_command_line_interface_t.F90#L519
```fortran
    if(present(error)) error = 0  ! <- New line
    if (self%is_parsed_) return
    ...
```

And finally, set `is_parsed_` variable to `.false.` in the `free()` procedure.

Side effects:

Nothing.