Developer kprintf's

[Samuel J. Greear]

We have quite a few kprintf's in the kernel that do not necessarily
report error conditions, they are purely informational and highlight
odd or rare cases that often the end-user of the operating system
really doesn't need to know about. This has been discussed quite a few
times on IRC.

I would like to put these under a sysctl like kern.developer and wrap
them in a function like dkprintf unless anyone has any major
objections. (I'm open to suggestions on the naming).

Sam