hyperv/hn: Change global tunable prefix to hw.hn

And use SYSCTL+CTLFLAG_RDTUN for them.

Suggested by: adrian
Reviewed by: adrian, Hongjiang Zhang <honzhan microsoft com>
Approved by: adrian (mentor)
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5274