systemctl: don't search in the full argv[0] for the invocation name

argv[0] might be prefixed by a path, and we shouldn't get confused by
that. Hence provide a simple helper call that abstracts the checking
away, which we can use everywhere, and expose the same behaviour, even
if argv[0] is not set.

(While we are at it, port all other multi-call binaries over to the new
helper, too)

Follow-up for: d41a9e4fc1

src/basic/process-util.c

@@ -1620,6 +1620,16 @@ int setpriority_closest(int priority) {
         return 0;
 }
 
+bool invoked_as(char *argv[], const char *token) {
+        if (!argv || isempty(argv[0]))
+                return false;
+
+        if (isempty(token))
+                return false;
+
+        return strstr(last_path_component(argv[0]), token);
+}
+
 static const char *const ioprio_class_table[] = {
         [IOPRIO_CLASS_NONE] = "none",
         [IOPRIO_CLASS_RT] = "realtime",