Use EC_GROUP_dup and EC_POINT_dup in EC_KEY_copy.

They do the same thing. This removes all callers of EC_GROUP_copy outside
EC_GROUP_dup.

Change-Id: I65433ee36040de79e56483dfece774e01e2e2743
Reviewed-on: https://boringssl-review.googlesource.com/3630
Reviewed-by: Adam Langley <agl@google.com>