unless anyone objects, i'll make kauth_cred_set*() return
a new credential and remove kauth_cred_copy.
ie. something like the following.

	kauth_cred_t
	kauth_cred_setuid(kauth_cred_t cred, uid_t uid)
	{
		kauth_cred_t newcred;

		if (cred->cr_uid == uid) {
			return cred;
		}
		if (cred->cr_refcnt == 1) {
			cred->cr_uid = uid;
			return cred;
		}
		newcred = kauth_cred_dup(cred);
		kauth_cred_free(cred);
		newcred->cr_uid = uid;
		return newcred;
	}

YAMAMOTO Takashi