vtnet, ptnet: include opt_*.h files early

Include opt_inet.h and opt_inet6.h early in the files including
virtio_net.h, since they use INET and/or INET6.
While there, remove redundant inclusion of sys/types.h, since it is
included already by sys/param.h.

There was a discussion to include opt_inet.h and opt_inet6.h also
in virtio_net.h. glebius suggested to add a mechanism for files
to check, if required opt_*.h files were included. virtio_net.h
will be the first consumer of this mechanism.

Reviewed by: glebius, Peter Lei
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52046