fix: dev: Stop using malloc_usable_size and malloc_size

The `malloc_usable_size()` can return size larger than originally allocated and when these sizes disagree the fortifier enabled by `_FORTIFY_SOURCE=3` detects overflow and stops the `named` execution abruptly.  Stop using these convenience functions as they are primary used for introspection-only.

Closes #4880

Merge branch '4880-dont-use-malloc_usable_size' into 'main'

Closes #4880

See merge request isc-projects/bind9!9400