Move authselect tmpfiles snippet into main tmpfiles snippet

Now that tmpfiles supports L? to only symlink things if the source
exists, let's make use of it to move the authselect stuff into the
main tmpfiles snippet.

Author: DaanDeMeyer

mkosi.conf.d/20-fedora/mkosi.extra/usr/lib/tmpfiles.d/etc-authselect.conf

@@ -35,3 +35,5 @@ L /etc/man_db.conf
 # Configuration for ldconfig
 L /etc/ld.so.conf
 L /etc/ld.so.conf.d
+# Required by authselect (Fedora/CentOS)
+L? /etc/authselect