[libc] Add Annex K strnlen_s function (#186112)
This patch adds the `strnlen_s` function from Annex K.
In order to reduce duplication between `strnlen` and `strnlen_s`, the
common logic has been extracted to a new internal function which both
now call.
In addition to the function definition, the patch adds a unit test and a
fuzzing test.
