Add a sysctl to allow disabling the monitoring thread.

Summary:
If a user uses powerd, or doesn't want to use the cycles monitoring, they can
now suspend the monitoring thread.

While here, reorganize the added prototypes to match existing groupings.

Reviewers: nwhitehorn, #powerpc, rpaulo

Reviewed By: #powerpc, rpaulo

Differential Revision: https://reviews.freebsd.org/D944
X-MFC-with:	r273009

MFC after:	3 weeks
Drop support for SSLv3.

MFC after:	1 week