[9.18] 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

Backport of MR !9400

Merge branch 'backport-4880-dont-use-malloc_usable_size-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9419