Make sched_random() return an unsigned number, and use uint32_t
consistently. This also matches the per-cpu pointer declaration
anyway.

This changes the tweak we give to the load from -32..31 to be 0..31
which seems more inline with the rest of the code (- rnd and the -=
64). It should also provide the randomness we need, and may fix a
signedness bug in the old code (it isn't clear that the effect was
intentional as opposed to sloppy, and the right shift of a signed
value is undefined to boot).

This stores sched_balance() behavior when it used random().

Differential Revision: https://reviews.freebsd.org/D1981