core: add support for expanding state/cache/log directory root in unit files

This augments %t which already resolves to the runtime directory root, and
should be useful for units that want to pass any of these paths in
command line arguments.

Example:

ExecStart=/usr/bin/mydaemon --datadir=%S/mydaemon

Why not expose a specifier resolving directly to the configured
state/runtime/cache/log dir? Three reasons:

1. Specifiers should be independent of configuration of the unit itself,
   and StateDirectory= and friends are unit configuration.  See
   03fc9c723c and related work.

2. We permit multiple StateDirectory= values per unit, and it hence
   wouldn't be clear which one is passed.

3. We already have %t for the runtime directory root, and we should
   continue with the same scheme.

man/systemd.unit.xml

@@ -1272,10 +1272,25 @@
           </row>
           <row>
       <entry><literal>%t</literal></entry>
-      <entry>Runtime directory</entry>
+      <entry>Runtime directory root</entry>
       <entry>This is either <filename>/run</filename> (for the system manager) or the path <literal>$XDG_RUNTIME_DIR</literal> resolves to (for user managers).</entry>
           </row>
           <row>
+      <entry><literal>%S</literal></entry>
+      <entry>State directory root </entry>
+      <entry>This is either <filename>/var/lib</filename> (for the system manager) or the path <literal>$XDG_CONFIG_HOME</literal> resolves to (for user managers).</entry>
+          </row>
+          <row>
+      <entry><literal>%C</literal></entry>
+      <entry>Cache directory root </entry>
+      <entry>This is either <filename>/var/cache</filename> (for the system manager) or the path <literal>$XDG_CACHE_HOME</literal> resolves to (for user managers).</entry>
+          </row>
+          <row>
+      <entry><literal>%L</literal></entry>
+      <entry>Logs directory root </entry>
+      <entry>This is either <filename>/var/log</filename> (for the system manager) or the path <literal>$XDG_CONFIG_HOME</literal> resolves to with <filename>/log</filename> appended (for user managers).</entry>
+          </row>
+          <row>
       <entry><literal>%u</literal></entry>
       <entry>User name</entry>
       <entry>This is the name of the user running the service manager instance. In case of the system manager this resolves to <literal>root</literal>.</entry>