During n2k20 we asked ourself why touching a header makes us rebuild a lot
of the kernel again. New year or new cycle maybe, that's time for a new
round of cleanup ;o)
Today I figured out why when I touch <sys/proc.h> I have to rebuild too
much. I came up with two cleanups, diff below is the first one.
<sys/kthread.h> is the only sys/ header including <sys/proc.h>. It is
not always needed and do not necessarily require including <sys/proc.h>.
The diff below removes it from the places where it isn't needed, replaces
it with <sys/proc.h> when that's what the file need, adds a forward
definition when it's enough and adds it in the places where a `proc' is