fetch: support EAI_ADDRFAMILY error, correct two error messages

With the change to return EAI_ADDRFAMILY from getaddrinfo(), fetch
would print "Unknown resolver error" for that error. Add that error
and its string to libfetch's table, using an #ifdef just in case.
Correct error strings for EAI_NODATA (although it is currently unused)
and EAI_NONAME. Should maybe rework the code to use gai_strerror(3),
but that doesn't map directly, and the current strings are shortened.

Reviewed in https://reviews.freebsd.org/D37139 with related changes.

Reviewed by: bz
MFC after: 1 month