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 https://lab.nexedi.com/nexedi/slapos/merge_requests/862#note_121470 and https://lab.nexedi.com/nexedi/slapos/merge_requests/862#note_121600.