add malloc_usable_size function and non-stub malloc.h

this function is needed for some important practical applications of
ABI compatibility, and may be useful for supporting some non-portable
software at the source level too.

I was hesitant to add a function which imposes any constraints on
malloc internals; however, it turns out that any malloc implementation
which has realloc must already have an efficient way to determine the
size of existing allocations, so no additional constraint is imposed.

for now, some internal malloc definitions are duplicated in the new
source file. if/when malloc is refactored to put them in a shared
internal header file, these could be removed.

since malloc_usable_size is conventionally declared in malloc.h, the
empty stub version of this file was no longer suitable. it's updated
to provide the standard allocator functions, nonstandard ones (even if
stdlib.h would not expose them based on the feature test macros in
effect), and any malloc-extension functions provided (currently, only
malloc_usable_size).