MINOR: polling: make epoll and kqueue not depend on maxfd anymore

Maxfd is really only useful to poll() and select(), yet epoll and
kqueue reference it almost by mistake :
  - cloning of the initial FDs (maxsock should be used here)
  - max polled events, it's maxpollevents which should be used here.

Let's fix these places.
diff --git a/src/fd.c b/src/fd.c
index b64130e..4397f6e 100644
--- a/src/fd.c
+++ b/src/fd.c
@@ -457,7 +457,7 @@
 int fork_poller()
 {
 	int fd;
-	for (fd = 0; fd <= maxfd; fd++) {
+	for (fd = 0; fd < global.maxsock; fd++) {
 		if (fdtab[fd].owner) {
 			fdtab[fd].cloned = 1;
 		}