sysctl.conf.5: Document rc.d/sysctl and rc.d/sysctl_lastload

Also, update the BUGS section. The example describes an issue, which is
not true anymore thanks to sysctl_lastload. Point readers to rcorder(8)
instead.

MFC after: 2 weeks

(cherry picked from commit 383ccaa4050e3cbbd8ce8163c8aebd07c3d94ab1)