Use void arguments list consistently in C code

On my way, fixed a couple of cases where we had `()` in the header, and
non-empty argument list in the source file.

PUBLISHED_FROM=5519526cf84e2bbd425a726fcc112fea1a95cbf1

examples/cookie_auth/cookie_auth.c

@@ -183,7 +183,7 @@ static void logout_handler(struct mg_connection *nc, int ev, void *p) {
 }
 
 /* Cleans up sessions that have been idle for too long. */
-void check_sessions() {
+void check_sessions(void) {
   double threshold = mg_time() - SESSION_TTL;
   for (int i = 0; i < NUM_SESSIONS; i++) {
     struct session *s = &s_sessions[i];