unwind: introduce stacktrace_walker

unwind: introduce stacktrace_walker

In current implementation, the stack trace is captured and printed at
the same time, in trace_syscall_exiting.  This approach cannot
provide user expected information when a system call changes the
memory mapping.  In such cases, the stack trace should be captured on
entering syscall and printed on exiting.

As the initial step for splitting capturing from printing, this change
introduces stacktrace_walker utility function.  It can be used both for
capturing in trace_syscall_entering and printing in
trace_syscall_exiting.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>