sys: Avoid relying on pollution from refcount.h

[markj]

Summary

Fix up headers which previously assumed that refcount.h includes
systm.h, and make them more self-contained. Then, replace the systm.h
include in refcount with kassert.h.

MFC after: 1 week

[imp]

We have tests to make sure that some (most) of the sys/*.h files are self-contained.
Can we remove anything from the 'known bad' lists after this?

[markj]

Remove refcount.h from badfiles.inc

[kib]

sys/sys/refcount.h

49

u_int comes from machine/atomic.h->sys/atomic_common.h->sys/types.h.
Do we want to rely on this, or should sys/types.h included there directly?

[markj]

Bring in sys/types.h for u_int

[markj]

sys/sys/refcount.h

49

I'd rather replace u_int with unsigned int, but it's a lot of churn.

[imp]

sys/sys/refcount.h

49

That might be only marginally better. You'd avoid the namespace pollution of types.h, since u_int isn't defined in _types.h.. But it would be a lot more churn...
So either choice is fine.