Lower WWAN DNS Priority

Fixes OS-issues#130 (closed)

The current DNS priority settings for WWAN were set far too low. Most connections (including WiFi) do not set DNS priority (set to 0) and per https://developer.gnome.org/NetworkManager/stable/nm-settings.html :

A lower value is better (higher priority). Zero selects a globally configured default value. If the latter is missing or zero too, it defaults to 50 for VPNs and 100 for other connections.

By setting both the "low" and "high" settings to 15 and 20 respectively, the WWAN DNS servers were always appearing above WiFi, even though WiFi had routing priority. This caused latency and other problems when the wwan connection was slow because the system would query those DNS servers before WiFi ones. Beyond that, it would even cause WWAN to override VPN DNS settings which isn't what we want.

This change puts the "low priority" setting above the default 100 that connections get when they don't otherwise set a priority, and the "high priority" slightly below 100. I did this instead of setting the values to 0 because I noticed that NM doesn't seem to be aware it should prioritize WiFi in that case so WWAN DNS servers were still sometimes taking precedence.