Bugfix: fll_execute does not execute full path.

Bugfix: fll_execute does not execute full path.

When passed a full path to a program (rather than depending on detecting in from just a program name) the program does not execute.

There are several logic flaws and mistakes.
- The last_slash should have "+1" to avoid including the slash itself.
- When environment is cleared, it need to potentially use "program" or "arguments.array[0].string" if a slash already exists in the provided name.
- The final NULL at the end of the program_path string is missing.
- The "program" or "arguments.array[0].string" should be used in general instead of always the "program" only.
- The fixated_is is being used incorrectly in private_fll_execute_path_arguments_fixate (and the documentation is incorrect).

I suspect that these functions are messy due to changes in design that were not fully updated (including the appropriate documentation).