Bug #124084 for IO-Async-Loop-Epoll: signal handler can be called recursively

In 0.17, the flag for signal pending is reset only after the call to the the handler. If the handler runs the loop (for example with calling ->get on some future), thus recursively calling loop_once, the handler can be also called recursively. Attached patch to reset the pending flag before the handler is being called. Currently I go around it by calling $loop->later(sub { .. handler body .. }) in the signal handler subroutine.