Rename krbtgt variable in KDC code

Rename krbtgt variable in KDC code

In a TGS request, the header ticket server is usually a local or
cross-realm TGS principal, but for ticket modification requests it
doesn't have to be.  Similarly, the server for an AS request is
usually a krbtgt principal, but in some cases it is not.  Since the
KDC code must consider all possibilities, avoid using the name
"krbtgt" for entries which aren't necessarily TGTs.

In process_tgs_req(), rename krbtgt to header_server and tgskey to
header_key.  In handle_authdata(), rename the parameters similarly and
pass NULL from process_as_req() for the header_server and header_key
parameters; the code which uses those parameters is adjusted to match.
In validate_transit_path(), rename krbtgt to header_srv.

Do not change the semantics of the sign_authdata DAL method at this
time, but more accurately document the krbtgt and krbtgt_key
parameters.