Refactor dnsglue functions using k5input

Refactor dnsglue functions using k5input

Use k5input to make the dnsglue.c and dnssrv.c functions simpler and
more robust.  Rename the dnsglue functions to use the now-preferred
k5_ prefix instead of krb5int_.  Use stdint type names instead of
unsigned char and unsigned short.  Remove some out-of-date comments.

Change the query loop in k5_dns_init() to continue expanding the
buffer if the reported answer length is exactly equal to the buffer
size, as that is the observed behavior from the glibc res_nquery()
when the buffer size is too small for the answer.