sysctl(3): Implement SYSCTL_FOREACH() to iterate all OIDs in a sysctl list.

To avoid using the sysctl list macros directly in external kernel modules.

Reviewed by: asomers, manu and asiciliano
Differential Revision: https://reviews.freebsd.org/D36748
Sponsored by: NVIDIA Networking

(cherry picked from commit c075ea46bca6d48ce9f639e4575077d06d392427)