[core] fall back to accept() if accept4() EPERM (fixes #2998)

On some architectures, accept4() might result in EPERM depending
on Linux kernel and/or glibc support for accept4()

(thx alex-che)

x-ref:
  "accept4 returns EPERM instead of ENOSYS on some platforms"
  https://redmine.lighttpd.net/issues/2998

src/fdevent.c

@@ -637,9 +637,18 @@ int fdevent_accept_listenfd(int listenfd, struct sockaddr *addr, size_t *addrlen
 					fd = -1;
 				}
 			}
-		} else if (errno == ENOSYS || errno == ENOTSUP) {
-			fd = accept(listenfd, addr, &len);
-			sock_cloexec = 0;
+		}
+		else {
+			switch (errno) {
+			case ENOSYS:
+			case ENOTSUP:
+			case EPERM:
+				fd = accept(listenfd, addr, &len);
+				sock_cloexec = 0;
+				break;
+			default:
+				break;
+			}
 		}
 	}
 	else {