Properly handle use_master in k5_init_creds_get

If we make multiple requests in an initial creds exchange, the
krb5_sendto_kdc call in k5_init_creds_get may flip the use_master
value from 0 to 1 if it detects that the response was from a master
KDC.  Don't turn this into a requirement for future requests during
the same exchange, or we may have trouble following AS referrals.
Reported by Sumit Bose.

ticket: 7650