krb5: Enable PRINC_LOOK_AHEAD in ksu

[cy]

Summary

PRINC_LOOK_AHEAD is the upstream default. Normally ksu determines the
target princiapl by (quoted from the man page)

a. default principal of the source cache

b. target_user@local_realm

c. source_user@local_realm

With PRINC_LOOK_AHEAD emabled, for each candidate in the above
list, select an authorized principal that has the same realm name
and first part of the principal name equal to the prefix of the
candidate. For example if candidate a) is [e-mail] and
jqpublic/[e-mail] is authorized to access the target account
then the default principal is set to jqpublic/[e-mail].

Case 2: source user is root.

If the target user is non-root then the default principal name
is target_user@local_realm. Else, if the source cache exists
the default principal name is set to the default principal of
the source cache. If the source cache does not exist, default
principal name is set to root\@local_realm.

Reported by: Dan Mahoney <[e-mail]>
MFC after: 3 days
MFC to: 15/stable

Test Plan

Running here locally.

[freebsd_gushi.org]

I'm original reporter (note, I use not-isc-email for Project stuff). I've confirmed this works and restores previous behavior as with heimdal ksu.