Make net/ifq.h C++ friendly

Don't use "new" as an identifier, and add explicit casts from void *.

As a general policy, FreeBSD doesn't make any C++ compatibility
guarantees for kernel headers like it does for userland, but it is a
small effort to do so in this case, to the benefit of a downstream
consumer (NetApp).

Reviewed by: rscheff
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27286