[krbdev.mit.edu #673] kadmin shows password when it shouldn't

[Tom Yu via RT]

This is interesting.  I'm not quite sure what causes this, and haven't
yet reproduced it, but I suspect something involving signal handling
and/or longjmp(), and possibly unfortunate interactions with stdio
buffers in fgets().  We may want to setvbuf() when reading passwords
with fgets().  Places to look at: util/ss/listen.c and
lib/krb5/os/read_pwd.c or lib/krb5/os/prompter.c (on the trunk).