support_format_addrinfo: Fix flags and canonname formatting

The address family splitting via format_ai_family made unpredictable
the place where the canonname field was printed.  This commit adjusts
the implementation so that the ai_flags is checked for consistency
across the list, and ai_canonname must only be present on the first
list element.

Tests for AI_CANONNAME are added to resolv/tst-resolv-basic.