"make install" unnecessarily creates certain directories

Creating etc/ is not necessary since 2009, when we started shipping bind.keys. That being said, IMHO it was unnecessary even before, but oh well.

Creating var/ is not necessary since 2008, which is when named_os_openfile() started calling mkdir().

The bottom line is that neither of these directories should be created by make install these days. I would fix it in v9.16 and leave ~"v9.11" alone, this is nothing critical.

(Found while working on #2629 (closed)/!4945 (merged).)