Rework r313352.
Rename kern_vm_* functions to kern_*. Move the prototypes to
syscallsubr.h. Also change Mach VM types to uintptr_t/size_t as
needed, to avoid headers pollution.
Requested by: alc, jhb
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D9535