Explicit register/unregister select APIs per event mode

Instead of registering and unregistering an fd to every operation
(read, write, execute), require the caller to explicitly register
a callback for an fd and an operation.
Updated CHANGES.txt for latest features
SelectTask: test case, bugfixes

- rename "execute" to "except".
- bugfix: During the last refactor, forgot to send the control NEWFD on
newly registered/unregisted fds
- Add a test case.  Verifies read/write signals, but not except events