This skeleton configuration file simply presents the (default empty)
NSD_EXTRA_OPTS variable to the user. The forthcoming OpenRC service
script will make use of this variable in exactly the same manner that
the systemd unit does.

This OpenRC service script is modified from the one currently used in
Gentoo Linux to start NSD. The main benefit of having it upstream
(i.e. here) is that several paths like @sbindir@ can be passed in
from the build system. Having it here should also avoid some
duplication of effort.
Currently the script is unusable because the build system is unaware
of it (and therefore doesn't make the necessary substitutions), but
that will change in a subsequent commit.

In autoconf-2.70 and later, there is a separate --runstatedir flag
that allows the user to specify the directory that would typically be
/run on Linux or /var/run on BSD. The line,
  AC_PREREQ([2.70])
is added to configure.ac to ensure that autoconf supports this; the
line
  runstatedir = @runstatedir@
is added to Makefile.in to allow the build system to make use of it.
The $(EDIT) command (which simply runs sed) was also updated to make
substitutions for @runstatedir@.

Use the pre-existing $(EDIT) command to make @foo@ substitutions in
contrib/nsd.openrc.in, producing contrib/nsd.openrc as output.

Both systemd and OpenRC can make use of tmpfiles.d entries as
described in
  https://www.freedesktop.org/software/systemd/man/latest/tmpfiles.d.html
If the user wants to use a local control socket, then a writable
temporary directory is needed to hold the socket. We cannot know
beforehand what the user's configuration will be, so this commit adds
a tmpfiles.d entry that unconditionally creates @runstatedir@/nsd and
makes it writable by the nsd user.
If the temporary directory turns out not to be needed, a superfluous
one is no big deal. But if the user does want to use a local socket,
then this saves him the trouble of securely creating a directory to
hold it.

The new example makes use of the location @runstatedir@/nsd that is
created by contrib/nsd-tmpfiles.conf and that will work "out of the
box."

Once again this simply uses the pre-existing $(EDIT) command to run
sed on the template file nsd-tmpfiles.conf.in and produce the output
file nsd-tmpfiles.conf.