kadmin's ktremove can remove wrong entries when removing kvno 0

kadmin's ktremove can remove wrong entries when removing kvno 0

Because of 8-bit wraparound, keytabs can contain entries with kvno 0.
Because 0 is a distinguished kvno value for krb5_kt_get_entry(),
kadmin's remove_principal() winds up substituting the specified kvno
with the highest-numbered kvno of the specified principal in the
keytab.  Make sure not to perform this substitution when in
specified-kvno mode.

(This fix leaves behind a very minor bug where "ktrem principal 0"
returns silently, instead of producing an error message like it
normally would, if principal exists in the keytab but not at kvno 0.)

ticket: 6854

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24611 dc483132-0cff-0310-8789-dd5450dbe970