[krbdev.mit.edu #2710] use BIND 8 parsing API

[Tom Yu via RT]

We should be using the BIND 8 parsing API (ns_initparse, etc.) rather
than relying on the HEADER struct (which I think is BIND 4). 
Apparently, using -DBIND_8_COMPAT to make the HEADER struct visible can
turn off the res_nsearch() prototype, etc.  If we use the ns_* API on
BIND 8 and 9, we can stop using -DBIND_8_COMPAT.

Affected files are lib/krb5/os/dnssrv.c and lib/krb5/os/hst_realm.c.