Mark more nodes as CTLFLAG_MPSAFE or CTLFLAG_NEEDGIANT (16 of many)

r357614 added CTLFLAG_NEEDGIANT to make it easier to find nodes that are
still not MPSAFE (or already are but aren’t properly marked).
Use it in preparation for a general review of all nodes.

This is non-functional change that adds annotations to SYSCTL_NODE and
SYSCTL_PROC nodes using one of the soon-to-be-required flags.

Mark all obvious cases as MPSAFE.

Reviewed by: royger
Approved by: kib (mentor, blanket)
Differential Revision: https://reviews.freebsd.org/D23638