util: rework strappenda(), and rename it strjoina()

After all it is now much more like strjoin() than strappend(). At the
same time, add support for NULL sentinels, even if they are normally not
necessary.

src/core/execute.c

@@ -1582,9 +1582,9 @@ static int exec_child(
 
                 if (context->private_tmp && runtime) {
                         if (runtime->tmp_dir)
-                                tmp = strappenda(runtime->tmp_dir, "/tmp");
+                                tmp = strjoina(runtime->tmp_dir, "/tmp");
                         if (runtime->var_tmp_dir)
-                                var = strappenda(runtime->var_tmp_dir, "/tmp");
+                                var = strjoina(runtime->var_tmp_dir, "/tmp");
                 }
 
                 r = setup_namespace(
@@ -2588,7 +2588,7 @@ void exec_command_dump(ExecCommand *c, FILE *f, const char *prefix) {
         assert(f);
 
         prefix = strempty(prefix);
-        prefix2 = strappenda(prefix, "\t");
+        prefix2 = strjoina(prefix, "\t");
 
         cmd = exec_command_line(c->argv);
         fprintf(f,