Some qp-trie operations will need to know the maximum number of labels in a name, so I wanted a standard macro definition with the right value. Replace DNS_MAX_LABELS from <dns/resolver.h with DNS_NAME_MAXLABELS in <dns/name.h>, and add its counterpart DNS_NAME_LABELLEN. Use these macros in `name.c` and `resolver.c`. Fix an off-by-one error in an assertion in `dns_name_countlabels()`.