resolved: increase most label buffers to fit a trailing NUL byte

resolved: increase most label buffers to fit a trailing NUL byte

This is just paranoia. In all these cases we don't really care about the
trailing NUL byte. But if there's space for it dns_label_unescape() is
going to insert it, and that's a good safety strategy.

This is a follow-up to c29c3adefa8cd859f8cb87d9ad62f3d77b7cd102 which
fixed an actual bug, unlike this commit, which is just paranoia.