libbsdmalloc: Fix sense of assertion in aligned_alloc.

Make mstats() decl match prototype so this compiles with -DMSTATS.

libbsdmalloc: Use a multiply-overflow that clang supports too.

XXX pullup-10

libbsdmalloc: Make aligned_alloc(A, S) actually align when A > S.

XXX pullup-10

libbsdmalloc: Lift C11 divisibility restriction on aligned_alloc.

Restriction was lifted in C17.

XXX pullup-10

libbsdmalloc: Fix build with DEBUG.

Nix __P while here.

XXX pullup-10

libbsdmalloc: Nix K&R definitions. Bump WARNS to 3.

libbsdmalloc: Nix trailing whitespace.

No functional change intended.

XXX pullup-10

libbsdmalloc: Provide all allocator front ends and fork hooks.

Front ends:

- aligned_alloc
- calloc
- posix_memalign

Fork hooks:

- _malloc_prefork
- _malloc_postfork
- _malloc_postfork_child

Oth

libbsdmalloc: Provide all allocator front ends and fork hooks.

Front ends:

- aligned_alloc
- calloc
- posix_memalign

Fork hooks:

- _malloc_prefork
- _malloc_postfork
- _malloc_postfork_child

Otherwise these will pull in the jemalloc definitions from libc,
which (a) defeats the purpose, and (b) won't work correctly with
fork and threads.

Thanks to kre@ and the thread on tech-userlevel for pointing me in
the right direction to making this actually work to override
jemalloc:

https://mail-index.netbsd.org/tech-userlevel/2023/06/30/msg013957.html

Note: libbsdmalloc doesn't set errno=ENOMEM on malloc failure, but
these front ends do (even aligned_alloc, which is from C11, which
doesn't define ENOMEM at all, but this pacifies our aligned_alloc
tests in t_posix_memalign.c). Might want to fix that.

XXX pullup-10

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Move UCB-licensed code from 4-clause to 3-clause licence.

Patches provided by Joel Baker in PR 22280, verified by myself.

Adding libbsdmalloc which is Chris Kingsley's `power of two bucket' malloc.

Suggested by christos.