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.
windows: Correctly interpret skipped tests, enable uhashlib and ubinascii