[krbdev.mit.edu #1109] setting default_keytab_name does not work with ktadd in kadmin

[Greg Hudson via RT]

It appears that Ken fixed this by accident in krb5 1.7 when performing a 
minor code cleanup (r20594).  FILE keytabs now support write operations, 
and WRFILE is redundant.

There are some remaining opportunities for code cleanup, and also a 
display error (kadmin's xst command displays the wrong keytab name if you 
override the default name).