srvconvert should use krb5_425_conv_principal_ext

[Derrick J Brashear <shadow@dementia.org>]

so it can provide a verifier function which knows the hostname of the
local host and can know that even though my realm is ANDREW.CMU.EDU 
rcmd.sun4 should become host/sun4.wash.acs.cmu.edu 

this could be done using hardcoded tables in krb5.conf, but that seems
pointless

it could be done in ..._ext using the resolve option, but given that we
can just look at the hostname of the local machine easily enough, there's
no reason for that.

i have a few other things in the order of 425 conversion to do but if this
sounds good i'll write it. i also have a verification function to
contribute which attempts verification by attempting to get a ticket for
the service, if anyone wants it.

-D