When NULL is passed into krb_get_in_pw_tkt, we only want to prompt once

When NULL is passed into krb_get_in_pw_tkt, we only want to prompt once
for the password rather than passing NULL along to each string to key
function causing each to prompt independently.  Modify krb_get_in_pw_tkt
to call des_read_pw_string directly and then pass the resulting password
into each string to key function as needed.  Add a prototype of
des_read_pw_string to krb4int.h since it's an exported function of
libdes425 but isn't prototyped in des.h.

Ticket: 2648
Version_Reported: 1.3.3
Component: krb5-libs

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@18129 dc483132-0cff-0310-8789-dd5450dbe970