unwind: resolve symbols through libunwind

The libunwind-based unwind code used elfutils to obtain function names.
This patch adds fallback to symbol resolution provided by libunwind if
elfutils do not return any result. This is useful on systems where
binaries have embedded minidebuginfo and libunwind has minidebuginfo
support while elfutils do not (e.g. fedora 19).

Related to #82.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Signed-off-by: Michal Toman <mtoman@redhat.com>