Implement Cache-Control: stale-if-error

[jech]

This is defined in RFC 5861, and should be fairly easy to implement in Polipo, since all the infrastructure is already there.

Note that there is a patch floating around the network that claims to do that[1], but, contrary to what the commit message claims, it doesn't do anything useful (no, Virginia, Polipo doesn't treat unknown Cache-Control directives as no-cache).

[1] https://industry.youview.com/resources/opensource/patches/polipo-1.0.4.1/0004-RFC5861-control-values-parsing.patch