Mark more nodes as CTLFLAG_MPSAFE or CTLFLAG_NEEDGIANT (18 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. All entries that haven't been marked
as MPSAFE before are by default marked as NEEDGIANT

Reviewed by: cem
Approved by: csprng, kib (mentor, blanket)
Differential Revision: https://reviews.freebsd.org/D23841