Define DNS_NAME_MAXLABELS and DNS_NAME_LABELLEN

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().