Respond to change in resolv.conf when using the system resolver
When using Polipo's own (asynchronous) DNS implementation, Polipo
still only reads resolv.conf once, upon startup.

Taken from the patch at https://industry.youview.com/resources/opensource/patches/polipo-1.0.4.1/0011-DEVARCH-4284-HTTP-Cache-Polipo-responds-to-change-in.patch