sys/net: add a new ether_vlanid_t type

ether_vlanid_t is a type to represent a VLAN ID, for example inside a
.1q tag. since this is specific to Ethernet, put it in net/ethernet.h.

change bridge to use the new type instead of uint{16,32}_t.

Reviewed by: adrian, kp
Differential Revision: https://reviews.freebsd.org/D49836