BUG/MINOR: listener: also clear the error flag on a paused listener

BUG/MINOR: listener: also clear the error flag on a paused listener

When accept() fails because a listener is temporarily paused, the
FD might have both FD_POLL_HUP and FD_POLL_ERR bits set. While we do
not exploit FD_POLL_ERR here it's better to clear it because it is
reported on "show fd" and is confusing.

This may be backported to all versions.

(cherry picked from commit 20aeb1c7cd38907d704a4d769695b9ea264fa4c0)
Signed-off-by: Willy Tarreau <w@1wt.eu>