[krbdev.mit.edu #7195] kill off last users of WRFILE kt_ops

[Benjamin Kaduk via RT]

src/lib/krb5/keytab/kt_file.c declares a _krb5_kt_ops struct for WRFILE
which "should no longer be needed, but is effectively aliased to FILE
for compatibility."  However, we still produce names with the WRFILE:
prefix at a few places in the tree -- we should change those to FILE: so
that WRFILE is actually not needed for anything in-tree.