malloc(9): extend contigmalloc(9) by a "slab cookie"
    
    Extend kern_malloc.c internals to also cover contigmalloc(9) by a
    "slab cookie" and not just malloc/malloc_large.  This allows us to
    call free(9) even on contigmalloc(9) addresses and deprecate
    contigfree(9).  Update the contigmalloc(9) man page accordingly.
    
    The way this is done (free(9) working for contigmalloc) will hide the
    UMA/VM bits from a consumer which may otherwise need to know whether
    the original allocation was by malloc or contigmalloc by looking at
    the cookie (likely via an accessor function).  This simplifies
    the implementation of consumers of mixed environments a lot.
    
    This is preliminary work to allow LinuxKPI to be adjusted to better
    play by the rules Linux puts out for various allocations.
    Most of this was described/explained to me by jhb.
    
    One may observe that realloc(9) is currently unchanged (and contrary
    to [contig]malloc/[contig]free an implementation may need access
    the "slab cookie" information given it will likely be implementation
    dependent which allocation type to use if size changes beyond the
    usable size of the initial allocation).
    
    Described by:	jhb
    Sponsored by:	The FreeBSD Foundation
    Reviewed by:	markj, kib
    Differential Revision: https://reviews.freebsd.org/D45812
    
    (cherry picked from commit 9e6544dd6e02c46b805d11ab925c4f3b18ad7a4b)