Now anyone can schedule events and get a callback to work as long
as the user data structure that is added for the event begins
with a kore_event data structure.
All event state is now kept in that kore_event structure and renamed
CONN_[READ|WRITE]_POSSIBLE to KORE_EVENT_[READ|WRITE].
Mimics how the header files are installed on a system
as PREFIX/include/kore.
This is required for getting kodev to use the headers from the
kore_source option instead of requiring the kore headers to be
installed on the system even when building as a single_binary.