CVE-2016-3706: getaddrinfo: stack overflow in hostent conversion [BZ #20010]

When converting a struct hostent response to struct gaih_addrtuple, the
gethosts macro (which is called from gaih_inet) used alloca, without
malloc fallback for large responses.  This commit changes this code to
use calloc unconditionally.

This commit also consolidated a second hostent-to-gaih_addrtuple
conversion loop (in gaih_inet) to use the new conversion function.

(cherry picked from commit 4ab2ab03)