Improve seperation between kernel code and userland code by requiring

Improve seperation between kernel code and userland code by requiring
that source files that #include kernel headers also #define _KERNEL or
_KERNEL_STRUCTURES as appropriate.

With-suggestions-from: joerg
Still-todo: systimer.h, and clean up scsi_da.c
Tested-by: cd /usr/src/nrelease && make installer_release
Approved-by: dillon
If-anything-breaks-yell-at: cpressey