<sys/types.h>: Make boolean_t kernel only type.

<sys/types.h>: Make boolean_t kernel only type.

 This effectively eliminates boolean_t from use in userland.  Injecting
 this type into userland codes often creates issues in DPorts where ports
 are defining their own boolean_t type.  Kernel headers now do not make
 any references to this type even under _KERNEL_STRUCTURES.
 If visibility is desired kernel headers can use __boolean_t that will
 not conflict with 3rd-party software codes.

 While there, make "struct pagerops" in vm/vm_pager.h kernel only, it is
 only used below for pagertab that is already under _KERNEL.