getnameinfo: Return EAI_OVERFLOW in more cases [BZ #19787]

The AF_LOCAL and AF_INET/AF_INET6 non-numerci service conversion
did not return EAI_OVERFLOW if the supplied buffer was too small,
silently returning truncated data.  In the AF_INET/AF_INET6
numeric cases, the snprintf return value checking was incorrect.

(cherry picked from commit 06674678)