This is another entry in the series to make more things
"const-clean" with the ultimate goal of merge_dive() take
const pointers.
This concerns functions taking pointers to events and
the fallout from making these const.
The somewhat debatable part of this commit might be
that get_next_event() is split in a two distinct
(const and non-const) versions with different names,
since C doesn't allow overloading. The linker should
recognize that these functions are identical and remove
one of them.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>