test,systemctl,nspawn: use "const char*" instead of "char*" as iterator for FOREACH_STRING()

The macro iterates through literal strings (i.e. constant strings),
hence it's more correct to have the iterator const too.

src/nspawn/nspawn.c

@@ -1964,11 +1964,11 @@ static int setup_hostname(void) {
 }
 
 static int setup_journal(const char *directory) {
-        sd_id128_t this_id;
         _cleanup_free_ char *d = NULL;
-        const char *p, *q;
+        const char *dirname, *p, *q;
+        sd_id128_t this_id;
+        char id[33];
         bool try;
-        char id[33], *dirname;
         int r;
 
         /* Don't link journals in ephemeral mode */

src/systemctl/systemctl.c

@@ -6933,9 +6933,9 @@ static int run_editor(char **paths) {
         if (r < 0)
                 return r;
         if (r == 0) {
-                char **editor_args = NULL, **tmp_path, **original_path, *p;
+                char **editor_args = NULL, **tmp_path, **original_path;
                 size_t n_editor_args = 0, i = 1, argc;
-                const char **args, *editor;
+                const char **args, *editor, *p;
 
                 argc = strv_length(paths)/2 + 1;
 

src/test/test-execute.c

@@ -131,7 +131,7 @@ static bool check_user_has_group_with_same_name(const char *name) {
 }
 
 static bool is_inaccessible_available(void) {
-        char *p;
+        const char *p;
 
         FOREACH_STRING(p,
                 "/run/systemd/inaccessible/reg",