sh: Keep ignored SIGINT/SIGQUIT after set in a background job

sh: Keep ignored SIGINT/SIGQUIT after set in a background job

If job control is not enabled, a background job (... &) ignores SIGINT and
SIGQUIT, but this can be reverted using the trap builtin in the same shell
environment.

Using the set builtin to change options would also revert SIGINT and SIGQUIT
to their previous dispositions.

This broke due to r317298. Calling setsignal() reverts the effect of
ignoresig().

Reported by:	bdrewery
MFC after:	1 week