Remove declaration of sa_handler if sa_sigaction is defined
Remove declaration of sa_handler if sa_sigaction is defined

On some platforms, sa_sigaction is implemented as a union with
sa_handler. Remove the silly assign to NULL.

In the cases where it's not defined, C99 struct declaration ensures
that sa_handler is set to NULL.

worr/wsh,elentar/wsh,worr/wsh,worr/wsh,elentar/wsh,elentar/wsh