Delete preevent copy constructors.

Copying a preevent is unreliable because it acts unlike copying
an event. So don't do it.
Remove unused.
Ensure `tamer -l` is updated when libraries are updated.
Add support for TAMER_PREFER_PREEVENT.

If TAMER_HAVE_PREEVENT && TAMER_PREFER_PREEVENT, then the same behavior
is provided as before recent commits: make_event(r) makes a preevent
when it can.