Reimplement krb5_get_in_tkt_with_skey in terms of krb5_get_init_creds,

Reimplement krb5_get_in_tkt_with_skey in terms of krb5_get_init_creds,
similar to how the password and keytab equivalents were done.

Eliminate krb5_get_in_tkt.  It's been very hard to use since we made
krb5_kdc_rep_decrypt_proc private (in krb5 1.7 the prototype was taken
out of krb5.h altogether), and it's unlikely that anything would have
used it directly in the first place.

Remove and/or simplify a lot of code depended on by krb_get_in_tkt,
including all of preauth.c.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24012 dc483132-0cff-0310-8789-dd5450dbe970