Expose the dns.jsdns.autoResetServerAlive setting and/or change the default value

[lieser]

With an unstable internet connection, the default behavior of the JavaScript DNS library resolver results in at some point marking all DNS servers as unreachable, even if it was only a temporary problem.
To get the DNS resolution to work again, one has to either manually reset the status (by changing the DNS settings or restarting TB), or change a hidden setting.

The default value should be reconsidered, and the setting probably be exposed in the GUI.

Related: #90

[lieser]

I changed the default to rest the alive status, but decided against making it configurable.

The initial (bad) idea behind the old default behavior was to prevent unnecessary DNS queries if they all will fail anyway because e.g. the network is down.
This should now be better handled with the support of the offline mode (#129). So hopefully there is no longer any use case for the old behavior.