MEDIUM: epoll: skip reports of stale file descriptors

MEDIUM: epoll: skip reports of stale file descriptors

Now that we can see that some events are reported for older instances
of a file descriptor, let's skip these ones instead of reporting
dangerous events on them. It might possibly qualify as a bug if it
helps fixing strange issues in certain environments, in which case it
can make sense to backport it along with the following recent patches:

  DEBUG: fd: add a counter of takeovers of an FD since it was last opened
  MINOR: fd: add a generation number to file descriptors
  DEBUG: epoll: store and compare the FD's generation count with reported event

(cherry picked from commit 8235a24782e528b9bf8ca9dd69c0a147556dfcb5)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit cdb4027d54baab39347bf2e1da48aff7d1af5429)
Signed-off-by: Willy Tarreau <w@1wt.eu>