core: Signal the event pipe at most one time

core: Signal the event pipe at most one time

This change will ensure that the event pipe is only signalled
at most one time during the course of any incoming events. New
events that occur while the event pipe is in the signalled state
will not cause the event pipe to be signalled again.

This change will provide support for the use of native events on
the Windows/WinCE backends, as these events are binary and do not
"count" the number of times they are signalled.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>