Include krb5_cc_copy_creds function

[luisfdez]

We are porting some code that needs to generate FILE caches while working a new system that is configured with KCM.

Exposing this function would be handy in this situation to perform actions like:

use Authen::Krb5;

Authen::Krb5::init_context();

my $credCache = Authen::Krb5::cc_default();
my $newCreds = Authen::Krb5::cc_resolve("FILE:/tmp/legacy.cc");

$newCreds->initialize($credCache->get_principal());
Authen::Krb5::cc_copy_creds($credCache, $newCreds);

[coveralls]

Coverage remained the same at 31.25% when pulling b897862 on luisfdez:feature/cc_copy into 3ad0ba2 on sophos:master.