fs_epoll: serveral epoll issues fix

1. fs_epoll: try again when epoll_teardown() return 0
when poll_notify() called larger than twice when epoll_wait() blocked
in the eph->sem, the semcount will be larger than 1 when epoll_wait()
unblocked and will return 0 directly at the next epoll_wait.
So retry to wait the eph->sem again when epoll_teardown return 0.

2. fs_epoll: poll_setup the fd again even this fd got non-expected event
Some poll implementations need call poll_setup again when their internal
states changed (e.g., local socket), so should add the fd to the epoll
teardown list and poll_setup again at the next epoll_wait even this fd
got the user non-expected event.

Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>

include/sys/poll.h

@@ -61,6 +61,11 @@
  *     Device has been disconnected (revents only).
  *   POLLNVAL
  *     Invalid fd member (revents only).
+ *
+ *   POLLALWAYS
+ *     Indicate that should ALWAYS call the poll callback whether the
+ *     drvier notified the user expected event or not, and this value is
+ *     used inside kernal only (events only).
  */
 
 #define POLLIN       (0x01)  /* NuttX does not make priority distinctions */
@@ -78,6 +83,8 @@
 #define POLLRDHUP    (0x10)  /* NuttX does not support shutdown(fd, SHUT_RD) */
 #define POLLNVAL     (0x20)
 
+#define POLLALWAYS   (0x10000) /* For not conflict with Linux */
+
 /****************************************************************************
  * Public Type Definitions
  ****************************************************************************/