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).

src/sysusers/sysusers.c

@@ -1359,10 +1359,12 @@ static bool item_equal(Item *a, Item *b) {
 static int parse_line(const char *fname, unsigned line, const char *buffer) {
 
         static const Specifier specifier_table[] = {
-                { 'm', specifier_machine_id, NULL },
-                { 'b', specifier_boot_id, NULL },
-                { 'H', specifier_host_name, NULL },
+                { 'm', specifier_machine_id,     NULL },
+                { 'b', specifier_boot_id,        NULL },
+                { 'H', specifier_host_name,      NULL },
                 { 'v', specifier_kernel_release, NULL },
+                { 'T', specifier_tmp_dir,        NULL },
+                { 'V', specifier_var_tmp_dir,    NULL },
                 {}
         };