ifconfig: Add an allmulti verb

Similar to "promisc", this allows the IFF_ALLMULTI flag to be toggled
from userspace if it happens to be useful to disable multicast packet
filtering. One use-case is when implementing IPv6 neighbour discovery
over netmap.

Reviewed by: zlei, glebius
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D46525