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)