print stack trace after each syscall

This patch prints the stack trace of the traced process after
each system call when using -k flag. It uses libunwind to
unwind the stack and to obtain the function name pointed by
the IP.

Tested on Ubuntu 12.04 64 with the distribution version of
libunwind (0.99-0.3ubuntu1) and on CentOS 6.3 with libunwind
form the source code. On Ubuntu you need the libunwind and
libunwind-dev package to compile the stack trace feature in
the code.

This is still a development patch, please let me know if you
would consider pulling this patch and if so what do you think
should be modified (I will also write man page and better
autoconf script).

Some of this code was originally taken from strace-plus of
Philip J. Guo.