Move config files to /usr/lib/systemd

... (e.g. /etc/systemd/system.conf → /usr/lib/systemd/systemd.conf).
Both config file locations were already supported, and the files
installed in /etc/ were "empty" (i.e. they had only comments and section
headers). The move does not change the configuration, but just makes
/etc more empty by default. See
https://github.com/systemd/systemd/commit/6495361c7d for more
discussion and details.

systemd.spec

@@ -662,6 +662,7 @@ CONFIGURE_OPTS=(
         # https://fedoraproject.org/wiki/Changes/Shorter_Shutdown_Timer
         -Ddefault-timeout-sec=45
         -Ddefault-user-timeout-sec=45
+        -Dconfigfiledir=/usr/lib
         -Doomd=true
         -Dadm-gid=4
         -Daudio-gid=63