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.

+ Non-author changes to origial patch:

  * Reformatted as per https://github.com/jech/polipo/commit/a697780c1b38b57caaab0b5fab23c45767f8902e