network-ndisc: avoid VLAs (#3725)

network-ndisc: avoid VLAs (#3725)

Do not allocate objects of dynamic and potentially large size on the stack
to avoid both clang compilation errors and unpredictable runtime behavior
on exotic platforms. Use the heap for that instead.

While at it, refactor the code a bit. Access 's->domain' via
NDISC_DNSSL_DOMAIN(), and refrain from allocating 'x' independently, but
rather reuse 's' if we're dealing with a new entry to the set.

Fixes #3717