Improve the user experience of backtraces (?): Do not display the bottom of the backt...

Improve the user experience of backtraces (?): Do not display the bottom of the backtrace (under main()); do not display whether it is a static or dynamic symbol; avoid to print garbage when one function pointer is equal to '(nil)' (even if I think that the libc should never give us such a stupid value, it's not a reason to fail miserably)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@3422 48e7efb5-ca39-0410-a469-dd3cf9ba447f