auditd(8): fix long-standing uninitialized memory use bug

The bogus use could lead to an infinite loop depending on how fast the
audit_warn script to execute.

By fixing read(2) interruptibility, d060887 (r335899) revealed another bug
in auditd_wait_for_events. When read is interrupted by SIGCHLD,
auditd_reap_children will always return with errno set to ECHILD. But
auditd_wait_for_events checks errno after that point, expecting it to be
unchanged since read. As a result, it calls auditd_handle_trigger with bogus
stack garbage. The result is the error message "Got unknown trigger 48." Fix
by simply ignoring errno at that point; there's only one value it could've
possibly had, thanks to the check up above.

The best part is we've had a fix for this for like 18 months and just never
merged it. Merge it now.

PR: 234209
Reported by: Marie Helene Kvello-Aune <freebsd AT mhka.no> (2018-12)
Submitted by: asomers (2018-07)
Reviewed by: me (in OpenBSM)
Obtained from: OpenBSM
X-MFC-With: r335899
Security: ¯\_(ツ)_/¯
Differential Revision: https://github.com/openbsm/openbsm/pull/45