libnetwork: macvlan: make configuration.fromOptions a constructor
libnetwork: macvlan: make configuration.fromOptions a constructor

This was effectively a constructor, but through some indirection; make it a
regular function, which is a bit more idiomatic.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

thaJeztah/docker,thaJeztah/docker,konstruktoid/docker-upstream,Mokolea/docker,moul/docker,nawawi/docker,trebonian/docker,moby/moby,konstruktoid/docker-upstream,jimjag/docker,nawawi/docker,Ramzec/docker,trebonian/docker,jimjag/docker,Mokolea/docker,moby/moby,cpuguy83/docker,moul/docker,cpuguy83/docker,cpuguy83/docker,jimjag/docker,thaJeztah/docker,Ramzec/docker,jimjag/docker,mahak/docker,Ramzec/docker,thaJeztah/docker,moby/moby,moul/docker,konstruktoid/docker-upstream,nawawi/docker,cpuguy83/docker,moul/docker,nawawi/docker,konstruktoid/docker-upstream,Mokolea/docker,mahak/docker,mahak/docker,trebonian/docker,Ramzec/docker,Mokolea/docker,mahak/docker,trebonian/docker,moby/moby