Simplify libkadm5 client realm initialization

The "realm" variable in init_any is used only to fill in the realm of
the service principal in gic_iter().  The service principal realm
should always be the realm we looked up config parameters for, so we
can supply that realm to get_init_creds() unconditionally and
eliminate the case where we use the client principal realm.

Also get rid of an outdated comment and an #if 0 block we will never
need again, and use SNPRINTF_OVERFLOW to check the snprintf result.
Allow realm in kadm5_init service names

Previously, if you passed a service name with a realm part to a
kadm5_init function, you would get a KRB5_PARSE_MALFORMED error
because the code would internally append its own '@realm' suffix
before parsing the name.  Fix this as follows:

Change gic_iter so instead of producing a full service name, it
produces a krb5_principal which is taken from the cred it acquires.
Pass the client and full service name around as principals, rather
than strings, and use the gss_nt_krb5_principal name type to import
them in setup_gss().  Don't append a realm to the input service name;
instead, pass the input service name directly to the gic functions
(which do not need a realm in the service name and will ignore the
realm if one is present).  For the INIT_CREDS case, parse the input
service name with KRB5_PRINCIPAL_PARSE_IGNORE_REALM and then set the
realm.

ticket: 7800