gaih_inet: split loopback lookup into its own function

Flatten the condition nesting and replace the alloca for RET.AT/ATR with
a single array LOCAL_AT[2].  This gets rid of alloca and alloca
accounting.

`git diff -b` is probably the best way to view this change since much of
the diff is whitespace changes.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: DJ Delorie <dj@redhat.com>
(cherry picked from commit 657472b2)