gpython/pymain: -O, sys.executable, ...

- Teach gpython and pymain about `-`;
- Teach gpython and pymain about -O;
- Teach pymain to always set sys.executable, so that subprocess and multiprocessing work correctly out of the box;
- Fix ./bin/gpython crash;
- Rewrite options parsing via incremental getopt-like thing.

This changes are motivated by nexedi/slapos!862 (comment 121470) and nexedi/slapos!862 (comment 121600).

/reviewed-by @jerome
/reviewed-on nexedi/pygolang!10