Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r271493,271688-271689,271696,271854,272139-272143:

Import HyperV Key-Value Pair (KVP) driver and daemon code by Microsoft,
many thanks for their continued support of FreeBSD.

While I'm there, also implement a new build knob, WITHOUT_HYPERV to
disable building and installing of the HyperV utilities when necessary.

The HyperV utilities are only built for i386 and amd64 targets.

Approved by: re (gjb)

Use a devd event to start hv_kvpd instead of doing so in rc.d script.
This is cleaner and eliminates the unneeded startup of KVP daemon on
systems that do not run as a Hyper-V guest.

Submitted by: hrs
X-MFC-with: 271493, 271688, 271699