Debugged version of patch worked up with Luke

Debugged version of patch worked up with Luke

Adds a callback to krb5int_sendto to examine the response and indicate
whether to quit the loop or not.  For sendto_kdc, keep going if the
returned error is "service unavailable".  Updated all other callers to
pass a null function pointer, which means to always break out of the
loop on any response (the old behavior).

ticket: 3334

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@19738 dc483132-0cff-0310-8789-dd5450dbe970