unwind: support unwinding from core hook

In-memory unwinding of dying processes, related to abrt/abrt#gh829. The
implementation mostly mirrors code in src/stack.c of elfutils repo
(jankratochvil/corepattern branch).

Related to abrt/abrt#829.

Signed-off-by: Martin Milata <mmilata@redhat.com>
debug unwinding from core hook using satyr binary

Write "|/usr/bin/satyr debug unwind-from-hook %e %i %s" to
/proc/sys/kernel/core_pattern in order to use this. Before kernel 3.18
the %i flag is not available - you can use %p for single-thread
processes.

Related to abrt/abrt#829.

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