if_vlan: allow vlan and vlanproto to be changed

It's currently not possible to change the vlan ID or vlan protocol (i.e.
802.1q vs. 802.1ad) without de-configuring the interface (i.e. ifconfig
vlanX -vlandev).
Add a specific flow for this, allowing both the protocol and id (but not
parent interface) to be changed without going through the '-vlandev'
step.

Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D35846