Implement address space guards.
Guard, requested by the MAP_GUARD mmap(2) flag, prevents the reuse of the allocated address space, but does not allow instantiation of the pages in the range. It is useful for more explicit support for usual two-stage reserve then commit allocators, since it prevents accidental instantiation of the mapping, e.g. by mprotect(2). Use guards to reimplement stack grow code. Explicitely track stack grow area with the guard, including the stack guard page. On stack grow, trivial shift of the guard map entry and stack map entry limits makes the stack expansion. Move the code to detect stack grow and call vm_map_growstack(), from vm_fault() into vm_map_lookup(). As result, it is impossible to get random mapping to occur in the stack grow area, or to overlap the stack guard page. Enable stack guard page by default. Reviewed by: alc, markj Man page update reviewed by: alc, bjk, emaste, markj, pho Tested by: pho, Qualys Sponsored by: The FreeBSD Foundation MFC after: 1 week Differential revision: https://reviews.freebsd.org/D11306 (man pages)