network: rename DiffServiceCodePoint to CopyDSCP

network: rename DiffServiceCodePoint to CopyDSCP

Old name was slightly misleading, because this flag does not determine
whether DSCP is used overall, but only if it is copied to the
decapsulated packet. Rename to better reflect that.

"Copy" does not imply direction. This is on purpose, because we might
later on enhance the setting to allow/disallow copying in the other
direction, to the encapsulated packet. If that is implemented,
CopyDSCP could understand additional values. This is nicer than
having two separate settings and follows the example of DHCP=.

Also, we try to avoid abbreviations, but we allow acronyms
like MTU, in DiscoverPathMTU=.

This setting was recently added, so it's fine to rename it without
backwards compat.