Add GSS_KRB5_NT_ENTERPRISE_NAME name type

Add GSS_KRB5_NT_ENTERPRISE_NAME name type

Add a new name-type OID which causes a string to be imported as an
enterprise name.  This is useful for authenticating and impersonating
users with their UPN names.

Resurrect t_imp_name test to exercise importing of the new name OID.
Also add a test using the new name in cross-realm protocol transition,
to exercise s4u_identify_user() with multiple realms.

[ghudson@mit.edu: added Windows export entry; adjusted comments and
test code; edited commit message]

ticket: 8756 (new)