pam_krb5: Restore allow_kdc_spoof option

[des]

Summary

Not only does the new pam_krb5 module not have the same allow_kdc_spoof
option that the old one had, its behavior in this matter defaults to
insecure. Reimplement allow_kdc_spoof and switch the default back.

[des]

Note that this patch updates the source for the documentation but not the mdoc file that we actually install; see D53885 for that.

[cperciva]

@des Please commit this and MFC to stable/15 ASAP so I can get it into 15.0-RC4 builds on Monday.

[des]

I am not committing this without the documentation, which has still not been approved.