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)

newlib/libc/sys/rtems/include/sys/mman.h

@@ -90,6 +90,7 @@
 /*
  * Extended flags
  */
+#define	MAP_GUARD	 0x00002000 /* reserve but don't map address range */
 #define	MAP_EXCL	 0x00004000 /* for MAP_FIXED, fail if address is used */
 #define	MAP_NOCORE	 0x00020000 /* dont include these pages in a coredump */
 #define	MAP_PREFAULT_READ 0x00040000 /* prefault mapping for reading */