Initial implementation of the debuginfo API

This patchset adds so_info and durin, which are used in debug info or
callsite analysis. The developer is expected to use the SO info API
documented in `include/babeltrace/so-info.h`. Durin is only used
internally by so_info as a wrapper around `libdw`.

The analysis itself is implemented in the new `lib/debuginfo.c`.

This introduces a dependency on `libelf` and `libdw` from
elfutils. The feature can be disabled at configure time with the
`--disable-debuginfo` flag.

Signed-off-by: Antoine Busque <abusque@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>