Chain signal handlers for SIGCHLD

For example, g_child_watch_source_new() calls ensure_unix_signal_handler_installed_unlocked(SIGCHLD) on Unix, that installs a signal handler on SIGCHLD, but that signal handler does not call the previously installed signal handler, breaking every program that uses its own process management and indirectly uses the glib library (for example the Gambas interpreter using webkit2gtk).