Configure fixed dns resolver list in baresip

There are often problems with dns resolver (no SRV Record support) and on Android >= 8.0 getting OS dns resolver from C code is not possible anymore.

So a fixed list of dns resolver in the baresip config would be a workaround:

Hurricane Electric - Anycast

• 74.82.42.42 *
• 2001:470:20::2

Quad9 without filtering - Anycast

• 9.9.9.10 *
• 2620:fe::10

Cloudflare - Anycast

• 1.1.1.1 *
• 2606:4700:4700::1111
• 1.0.0.1
• 2606:4700:4700::1001

*) Selected for v19.03.0