Add systemd unit file for nscd

Provide an example systemd unit and tmpfile for running nscd.
Add --enable-maintainer-mode configure option

Autoconf is tested for and run if needed only when --enable-maintainer-mode
is used on configure.  This results in the autom4te.cache directory only
being written in the source directory during configure if automatic
autoconf usage is requested.

Fixes BZ #14120.