Let's teach gpython and pymain about -O because buildout calls `python -O`
to byte-compile python sources in optimized mode:

nexedi/slapos!862 (comment 121470)

When seeing -O, we take the approach to reexecute underlying python with
-O, so that e.g.

	gpython -O file.py

becomes executed as

	python -O gpython file.py

This ensures correctness. The same approach could be used to implement
support for e.g. -S and other low-level options.

An earlier attempt to implement -O without reexecution can be seen at

nexedi/pygolang!7

together with list of problems that arise via that way.

Original idea to reexecute itself come from @jerome.