Fixes T3471. Specific issues:
- Add the ability to set a temporary cookie (expires when the browser closes).
- We overwrote 'phcid' on every page load. This creates some issues with browser extensions. Instead, only write it if isn't set. To counterbalance this, make it temporary.
- Make the 'next_uri' cookie temporary.
- Make the 'phreg' cookie temporary.
- Fix an issue where deleted cookies would persist after 302 (?) in some cases (this is/was 100% for me locally).