Do not reset SIGCHLD handler in tracees to SIG_DFL

Do not reset SIGCHLD handler in tracees to SIG_DFL

While strace resets SIGCHLD handler to the default action so that
waitpid definitely works without losing track of children, tracees
should not inherit this change.

* strace.c (struct exec_params): Add child_sa field.
(init): When setting SIGCHLD handler to SIG_DFL, save the old handler.
(exec_or_die): Restore SIGCHLD handler if it was different from SIG_DFL
at startup.
* NEWS: Mention this change.