Bring in MemGuard, a very simple and small replacement allocator
designed to help detect tamper-after-free scenarios, a problem more
and more common and likely with multithreaded kernels where race
conditions are more prevalent.

Currently MemGuard can only take over malloc()/realloc()/free() for
particular (a) malloc type(s) and the code brought in with this
change manually instruments it to take over M_SUBPROC allocations
as an example. If you are planning to use it, for now you must:

1. Put "options DEBUG_MEMGUARD" in your kernel config.
2. Edit src/sys/kern/kern_malloc.c manually, look for

	   "XXX CHANGEME" and replace the M_SUBPROC comparison with
	   the appropriate malloc type (this might require additional
	   but small/simple code modification if, say, the malloc type
	   is declared out of scope).

3. Build and install your kernel. Tune vm.memguard_divisor

	   boot-time tunable which is used to scale how much of kmem_map
	   you want to allott for MemGuard's use.  The default is 10,
	   so kmem_size/10.

ToDo:

1. Bring in a memguard(9) man page.
2. Better instrumentation (e.g., boot-time) of MemGuard taking

	   over malloc types.

3. Teach UMA about MemGuard to allow MemGuard to override zone

	   allocations too.

4. Improve MemGuard if necessary.

This work is partly based on some old patches from Ian Dowse.