- Make many variables local to the opcode; Kill u, v, w, and x.
- Force every opcode to end with DISPATCH or jump to error handling.
- Simplify error handling.
- Check error statuses in more places.

Closes #16191.

Add handling of the 6.2 release line, aka Windows 8 and Windows 2012 Server.

Content-Length header.  Patch by Antoine Pitrou.

fromlist which causes its own ImportError (e.g. the module tries to
import a non-existent module) should have that exception propagate.