adding execinfo.h

[christos%zoulas.com@localhost (Christos Zoulas)]

Hi,

I've written an implementation of the backtrace family of functions
described in:

    http://www.kernel.org/doc/man-pages/online/pages/man3/backtrace.3.html

You can find it in:

    http://www.netbsd.org/~christos/libexecinfo.tar.gz

In addition to providing support for resolving static symbols via libelf,
it also allows for formatting control of the output.

Should I commit it?

christos