Drop privileges when executing programs as needed

[JeremyRand]

The chainloader for the Namecoin Core and Dnssec-Trigger installers isn't dropping its Administrator privileges before launching the installers. This seems to be causing a variety of unfortunate behavior, including a symptom where Namecoin Core's installer thinks that its AppData folder is located in ProgramData (which then causes permission errors the next time Namecoin Core is launched from the start menu, even after you later uninstall and reinstall Namecoin Core). And of course it's also bad from a security point of view.

Bitcoin Core actually has the same issue in their NSIS script (when it launches bitcoin-qt.exe at the end). I just submitted a PR to Bitcoin Core to fix that issue; if it passes peer review I'll adapt it for ncdns-nsis and submit a PR here as well.

[JeremyRand]

The symptoms that were believed to be caused by this issue were actually fixed by #42 . We should still check whether we're executing anything with unnecessary privileges, though.