Add SIGCHLD to the list of signals we install sudo_handler() for.

Otherwise, it is possible for the command to exit before the SIGCHLD
handler is installed.  POSIX says that signals that are ignored by
default are still ignored even if the signal mask would block them.
We need to have a handler installed for SIGCHLD before the fork().

src/signal.c

@@ -127,7 +127,6 @@ init_signals(void)
 
     for (ss = saved_signals; ss->signo > 0; ss++) {
 	switch (ss->signo) {
-	    case SIGCHLD:
 	    case SIGCONT:
 	    case SIGPIPE:
 	    case SIGTTIN: