system(3): Fix brain glitch in previous commit

We were saving SIGINT twice instead of SIGINT and SIGQUIT.

Also restore original order of operations (SIGINT then SIGQUIT), which
matches the order in which they're discussed in the POSIX description
of system(3).

MFC after: 1 week
Sponsored by: Klara, Inc.
Fixes: 48368f702423 ("system(3): Address test robustness issue")