system(3): Address test robustness issue

[des]

Summary

Don't assume that SIGINT and SIGQUIT are set to SIG_DFL at the start
of the test. Instead, retrieve their current dispositions and verify
that they are restored at the end of the test.

MFC after: 1 week
Sponsored by: Klara, Inc.

[kevans]

lib/libc/tests/stdlib/system_test.c

134

Two points:

1.) I think we want another set of tests, then, and be explicit about it: one that explicitly sets them to SIG_DFL and one that ignores them, so that we can be sure no matter the test runner's configuration that it's restoring rather than that the disposition upon return was accidentally right

2.) We should probably use sigaction() for recording existing disposition if we suspect atf might have setup a handler to avoid breaking it

[des]

use sigaction

[kevans]

Agreed out-of-band that it's fine to defer testing both behaviors for the time being

[des]

const