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.

src/journal/meson.build

@@ -107,7 +107,7 @@ journalctl_sources = files('''
         pcre2-dlopen.h
 '''.split())
 
-if install_sysconfdir
+if install_sysconfdir_samples
         install_data('journald.conf',
                      install_dir : pkgsysconfdir)
 endif