Rework setting PTE_D for kernel mappings.

Rather than unconditionally setting PTE_D for all writeable kernel
mappings, set PTE_D for writable mappings of unmanaged pages (whether
user or kernel). This matches what amd64 does and also matches what
the RISC-V spec suggests (preset the A and D bits on mappings where
the OS doesn't care about the state).

Suggested by: alc
Reviewed by: alc, markj
Sponsored by: DARPA