Fix: miscellaneous memory handling fixes

Issues found with Valgrind:

- Uninitialized memory sent through sendmsg,
- Uninitialized realloc memory,
- Erroneous pointer arithmetic in jul_list_events(),
- Memory leaks in kernel_consumer_add_channel(),
  kernel_consumer_add_metadata(),
- Memory leak (snapshot_destroy missing hash table cleanup),
- Uninitialized poll/epoll memory on realloc.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>