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 slapos!862 (comment 121470) and slapos!862 (comment 121600).