core: introduce specifiers for /tmp and /var/tmp

This corresponds nicely with the specifiers we already pass for
/var/lib, /var/cache, /run and so on.

This is particular useful to update the test-path service files to
operate without guessable files, thus allowing multiple parallel
test-path invocations to pass without issues (the idea is to set $TMPDIR
early on in the test to some private directory, and then only use the
new %T or %V specifier to refer to it).

man/systemd.unit.xml

@@ -1626,6 +1626,11 @@
             <entry>Runtime directory root</entry>
             <entry>This is either <filename>/run</filename> (for the system manager) or the path <literal>$XDG_RUNTIME_DIR</literal> resolves to (for user managers).</entry>
           </row>
+          <row>
+            <entry><literal>%T</literal></entry>
+            <entry>Directory for temporary files</entry>
+            <entry>This is either <filename>/tmp</filename> or the path <literal>$TMPDIR</literal>, <literal>$TEMP</literal> or <literal>$TMP</literal> are set to.</entry>
+          </row>
           <row>
             <entry><literal>%u</literal></entry>
             <entry>User name</entry>
@@ -1641,6 +1646,11 @@
             <entry>Kernel release</entry>
             <entry>Identical to <command>uname -r</command> output</entry>
           </row>
+          <row>
+            <entry><literal>%V</literal></entry>
+            <entry>Directory for larger and persistent temporary files</entry>
+            <entry>This is either <filename>/var/tmp</filename> or the path <literal>$TMPDIR</literal>, <literal>$TEMP</literal> or <literal>$TMP</literal> are set to.</entry>
+          </row>
           <row>
             <entry><literal>%%</literal></entry>
             <entry>Single percent sign</entry>