Mark more nodes as CTLFLAG_MPSAFE or CTLFLAG_NEEDGIANT (2 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.

Reviewed by: hselasky, kib, zeising
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D23631