Add install-sysconfdir=no-samples option for (non-)installation of sample configs

By default, systemd installs various sample configuration files
containing commented-out defaults. Systems seeking to minimize the
number of files in /etc may wish to install directories and
configuration files that have semantic effects, but not install not
commented-out sample configuration files.

Turn install-sysconfdir into a multi-valued option, with a "no-samples"
value to skip installing sample-only configuration files.