stand: dev_net: correct net_open's interpretation of params

net_open previously casted the first vararg to a char * and this was
half-OK: at first, it is passed to netif_open, which would cast it back to
the struct devdesc * that it really is and use it properly. It is then
strdup()d and used as the netdev_name, which is objectively wrong.

Correct it so that the first vararg is properly casted to a struct devdesc *
and the netdev_name gets set properly to make it more clear at a glance that
it's not doing something horribly wrong.

Reported by: mmel
Reviewed by: imp, mmel, tsoome
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19206