tmpfiles: Implement L? to only create symlinks if source exists

This allows a single tmpfiles snippet with lines to symlink directories
from /usr/share/factory to be shared across many different configurations
while making sure symlinks only get created if the source actually exists.

test/units/TEST-22-TMPFILES.21.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# shellcheck disable=SC2235
+set -eux
+
+# Test L?
+
+rm -rf /tmp/tmpfiles
+
+root="/tmp/tmpfiles"
+mkdir "$root"
+touch "$root/abc"
+
+SYSTEMD_LOG_LEVEL=debug systemd-tmpfiles --create - --root=$root <<EOF
+L? /i-dont-exist - - - - /def
+L? /i-do-exist - - - - /abc
+EOF
+
+(! test -L "$root/i-dont-exist")
+test -L "$root/i-do-exist"