link-state change for virtual interfaces

[Rin Okuyama <rokuyama.rk%gmail.com@localhost>]

Hi,

Recently, link-state emulation has been added to shmif(4) and
vether(4). Unfortunately, different operations are required for
link-up/down for these interfaces:

For shmif(4)
- link-up:   ifconfig shmif%d media auto
- link-down: ifconfig shmif%d media none

https://mail-index.netbsd.org/source-changes/2024/08/20/msg152934.html
https://mail-index.netbsd.org/source-changes/2024/08/20/msg152935.html

For vether(4)
- link-up:   ifconfig vether%d link1
- link-down: ifconfig vether%d -link1

https://mail-index.netbsd.org/source-changes/2024/09/24/msg153517.html
https://mail-index.netbsd.org/source-changes/2024/09/24/msg153525.html

Wouldn't it be better to unify as far as possible? If so,
which one is more preferable? (The 3rd candidate may be
to add dedicated flag bit, but it seems too much to me.)

Thanks,
rin