tevent: Rename wakeup fds

tevent: Rename wakeup fds

This makes the reading end of the signalling pipe special: If we have eventfd,
this is the same as the write fd. Without eventfd, it will have to be a
separate fd. This moves the requirement to #ifdef from the writing end to the
reading end. Why? We'll use the writing end somewhere else too soon, and this
patch avoids an #ifdef in that new place.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>