ecore/signal: increase maximum signal throughput

ecore/signal: increase maximum signal throughput

this adds 4 more signal handling fds and loops over them for reading/writing
signal info in order to handle more signals when the buffer of one (or more)
pipes is full

also update the unit test to verify that we are receiving all the events without
dropping any and bump the number of signals to 2000 since we should now be able to
handle that many

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D10027