Accept anonymous GSS names in kadmind

The krb5 implementation of gss_display_name() reports the name type as
GSS_C_NT_ANONYMOUS if the client uses an anonymous principal.  Accept
this name type in gss_name_to_string and gss_to_krb5_name so that
anonymous kadmin can work.

Also improve code hygiene: call gss_name_to_string from
gss_to_krb5_name to reduce code repetition; use gss_oid_equal instead
of pointer comparison for name types; and don't assume that the
gss_display_name result buffer is zero-terminated.

ticket: 7740 (new)
target_version: 1.12
tags: pullup
Use correct default principal for kadmin -n

Use WELLKNOWN/ANONYMOUS@realm as the default principal for kadmin -n,
just like we do for kinit -n.

ticket: 7741 (new)
target_version: 1.12
tags: pullup
Add tests for anonymous kadmin