ifnet: Add handling for toggling IFF_ALLMULTI in ifhwioctl()

IFF_ALLMULTI has an associated activation counter and so needs special
treatment, like IFF_PROMISC. Introduce IFF_PALLMULTI, akin to
IFF_PPROMISC, which indicates that userspace requested allmulti mode,
and handle it specially in ifhwioctl().

Reviewed by: zlei, glebius
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D46524