Remove gratuitous copyouts of unchanged struct mac.

[brooks]

Summary

The get operations change the data pointed to by the structure, but do
not update the contents of the struct.

Mark mac_[gs]etsockopt_*label() const to prevent this from changing in
the future.

Also const poison mac_check_structmac_consistent().

[brooks]

• Rebase

[brooks]

Rebase

[markj]

Looks reasonable.