Add handling for appearing/disappearing of ingress addresses to if_gif(4).

* register handler for ingress address appearing/disappearing;
* add new srcaddr hash table for fast softc lookup by srcaddr;
* when srcaddr disappears, clear IFF_DRV_RUNNING flag from interface,
  and set it otherwise;
* remove the note about ingress address from BUGS section.

Sponsored by:	Yandex LLC
Differential Revision:	https://reviews.freebsd.org/D17134

Add handling for appearing/disappearing of ingress addresses to if_gre(4).

* register handler for ingress address appearing/disappearing;
* add new srcaddr hash table for fast softc lookup by srcaddr;
* when srcaddr disappears, clear IFF_DRV_RUNNING flag from interface,
  and set it otherwise;

Sponsored by:	Yandex LLC
Differential Revision:	https://reviews.freebsd.org/D17214

Add handling for appearing/disappearing of ingress addresses to if_me(4).

* register handler for ingress address appearing/disappearing;
* add new srcaddr hash table for fast softc lookup by srcaddr;
* when srcaddr disappears, clear IFF_DRV_RUNNING flag from interface,
  and set it otherwise;

Sponsored by:	Yandex LLC

Add the check that current VNET is ready and access to srchash is allowed.

This change is similar to r339646. The callback that checks for appearing
and disappearing of tunnel ingress address can be called during VNET
teardown. To prevent access to already freed memory, add check to the
callback and epoch_wait() call to be sure that callback has finished its
work.