It should be floor(DNS_NAME_MAXWIRE / 2) + 1 == 128
The mistake was introduced in c6bf51492d because:
* I was refactoring an existing `DNS_MAX_LABELS` defined as 127
* There was a longstanding bug in `dns_name_isvalid()` which
checked the number of labels against 127U instead of 128
* I mistakenly thought `dns_name_isvalid()` was correct and
`dns_name_countlabels()` was incorrect, but the reverse was true.
After this commit, occurrances of `DNS_NAME_MAXLABELS` with value
128 are consistent with the use of 127 or 128 before commit
c6bf51492d except for the mistake in `dns_name_isvalid()`.
This commit adds a test case that checks the MAXLABELS case
in `dns_name_fromtext()` and `dns_name_isvalid()`.
e8ff0f0c08