MFC:

There is an issue (not seen in our testing) where "yarrow" and
"dummy" switch priorities, and the users are left with no usable
/dev/random. The fix assigns priories to these and gives the users
what they want. The override tuneable has a stupid name (blame me!)
and this fixes it to be something that 'sysctl kern.random' emits
and is the right thing to set.

Approved by:	re (gjb)
Approved by:	secteam (cperciva)