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/shared/specifier.c

@@ -15,6 +15,7 @@
 #include "sd-id128.h"
 
 #include "alloc-util.h"
+#include "fs-util.h"
 #include "hostname-util.h"
 #include "macro.h"
 #include "specifier.h"
@@ -222,6 +223,40 @@ int specifier_user_shell(char specifier, void *data, void *userdata, char **ret)
         return get_shell(ret);
 }
 
+int specifier_tmp_dir(char specifier, void *data, void *userdata, char **ret) {
+        const char *p;
+        char *copy;
+        int r;
+
+        r = tmp_dir(&p);
+        if (r < 0)
+                return r;
+
+        copy = strdup(p);
+        if (!copy)
+                return -ENOMEM;
+
+        *ret = copy;
+        return 0;
+}
+
+int specifier_var_tmp_dir(char specifier, void *data, void *userdata, char **ret) {
+        const char *p;
+        char *copy;
+        int r;
+
+        r = var_tmp_dir(&p);
+        if (r < 0)
+                return r;
+
+        copy = strdup(p);
+        if (!copy)
+                return -ENOMEM;
+
+        *ret = copy;
+        return 0;
+}
+
 int specifier_escape_strv(char **l, char ***ret) {
         char **z, **p, **q;