For a long time the DNS address was the same as default gateway. This is
still the case in R3.x, but using `qubes-gateway` configuration
parameter for it is misleading. It should be up to dom0 to provide DNS
address (whether the value is the same as gateway or not).
FixesQubesOS/qubes-issues#1817
Get rid of underscores in filenames, use dashes instead.
This is first part of cleanup in filenames.
"qubes_rpc" still untouched - will be in separate commit.