unix/windows: Make sure that process exit code is portable 8-bit value.

This fixes FORCED_EXIT internal flag leaking into Windows exit code.
py: Add execfile function (from Python 2); enable in stmhal port.

Adds just 60 bytes to stmhal binary.  Addresses issue #362.
docs: Add CPy diff note for print_exception; embellish sys.platform.
py: Use sequence of strings for named tuple initialization

- remove single string initialization style
- take list of strings instead
- store list in the type for fast lookup
py: Allow keyword arguments for namedtuple