malloc: make malloc_large closer to standalone

[mjg]

Summary

This moves entire large alloc handling out of all consumers, apart from deciding to go there.

Apart from shortening malloc as it is it makes it feasible to replace the allocation code with smaller helpers to create a fast path.

[markj]

sys/kern/kern_malloc.c

592

Below it is ifdef DEBUG_REDZONE, would be nice to make it consistent.

[mjg]

• fix a redzone bug. malloc_dbg can bump the size over kmem_zmax, meaning it has to happen before checking for malloc_large