MFC r265798, r265815, r266091

Add a man page for the new vt.4 device.
MFC r262329:
Define the KM_NORMALPRI flag for kmem_alloc(), as it is used in some
upstream DTrace code.

MFC r262330:
1452 DTrace buffer autoscaling should be less violent

illumos/illumos-gate@6fb4854bed54ce82bd8610896b64ddebcd4af706