RFC5861 control values parsing
Merely parse (recognise) "stale-while-revalidate" and "stale-if-error"
Cache-Control values; Polipo's behaviour won't actually be affected by these
values, but before this change these values caused Polipo not to cache the
responses (because Polipo doesn't cache anything with Cache-Control values it
doesn't understand).

The behaviour of stale-if-error can be achieved by using Polipo's configuration
option RelaxTransparency=yes.

The original patch is here: https://industry.youview.com/resources/opensource/patches/polipo-1.0.4.1/0004-RFC5861-control-values-parsing.patch

+ Non-author changes to origial patch:

* Reformatted so as to apply to the 1.1.0 branch
Parse pre-check and post-check cache control values.
The rationale is basically the same as for the "RFC5861 control
values parsing" commit: merely recognise pre-check and post-check
cache control values so that the responses are cached by polipo.