Use OS DNS resolver via DNS.jsm

[lieser]

Thunderbird provides a DNS.jsm module, which seems to be able to do TXT DNS resolving via the OS provided libraries.

This should probably become the default resolver used by the add-on. Especially because the current default one has some problems (e.g. #136)

[ghost]

Does the module provide DNSSEC validation?

[lieser]

No, the module currently does not support DNSSEC.
Whether the underlying native libraries used (resolv on Linux/Mac, DnsAPI on Windows) do support it I don't know.

Note that it also still has a bug, that makes it currently not suitable to be used in the add-on: https://bugzilla.mozilla.org/show_bug.cgi?id=1583053