Add an ability accept encapsulated packets from different sources by one
gif(4) interface. Add new option "ignore_source" for gif(4) interface.
When it is enabled, gif's encapcheck function requires match only for
packet's destination address.

Differential Revision: https://reviews.freebsd.org/D2004
Obtained from: Yandex LLC
MFC after: 2 weeks
Sponsored by: Yandex LLC