nss_dns: In gaih_getanswer_slice, skip strange aliases (bug 12154)

If the name is not a host name, skip adding it to the result, instead
of reporting query failure.  This fixes bug 12154 for getaddrinfo.

This commit still keeps the old parsing code, and only adjusts when
a host name is copied.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>