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.

MFC after: 1 month
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D17134