sh: Allow trapping SIGINT/SIGQUIT after ignore because of '&'.

If job control is not enabled, background jobs started with  ... &  ignore
SIGINT and SIGQUIT so that they are not affected by such signals that are
intended for the foreground job. However, this should not prevent
reassigning a different action for these signals (as if the shell invocation
inherited these signal actions from its parent).

Austin group issue #751

Example:
  { trap - INT; exec sleep 10; } & wait
A Ctrl+C should terminate the sleep command.

bin/sh/trap.c

@@ -362,10 +362,12 @@ void
 ignoresig(int signo)
 {
 
+	if (sigmode[signo] == 0)
+		setsignal(signo);
 	if (sigmode[signo] != S_IGN && sigmode[signo] != S_HARD_IGN) {
 		signal(signo, SIG_IGN);
+		sigmode[signo] = S_IGN;
 	}
-	sigmode[signo] = S_HARD_IGN;
 }
 
 

tools/regression/bin/sh/builtins/trap13.0

@@ -0,0 +1,8 @@
+# $FreeBSD$
+
+{
+	trap 'exit 0' INT
+	${SH} -c 'kill -INT $PPID'
+	exit 3
+} &
+wait $!

tools/regression/bin/sh/builtins/trap14.0

@@ -0,0 +1,10 @@
+# $FreeBSD$
+
+{
+	trap - INT
+	${SH} -c 'kill -INT $PPID' &
+	wait
+} &
+wait $!
+r=$?
+[ "$r" -gt 128 ] && [ "$(kill -l "$r")" = INT ]