Rename mac_create_cred() MAC Framework entry point to mac_copy_cred(),
and the mpo_create_cred() MAC policy entry point to
mpo_copy_cred_label(). This is more consistent with similar entry
points for creation and label copying, as mac_create_cred() was
called from crdup() as opposed to during process creation. For
a number of policies, this removes the requirement for special
handling when copying credential labels, and improves consistency.

Approved by: re (scottl)
Obtained from: TrustedBSD Project
Sponsored by: DARPA, Network Associates Laboratories