Enable ROUTE_MPATH support in GENERIC kernels.

Ability to load-balance traffic over multiple path is a must-have thing for routers.
It may be used by the servers to balance outgoing traffic over multiple default gateways.

The previous implementation, RADIX_MPATH stayed in the shadow for too long.
It was not well maintained, which lead us to a vicious circle - people were using
non-contiguous mask or firewalls to achieve similar goals. As a result, some routing
daemons implementation still don't have multipath support enabled for FreeBSD.

Turning on ROUTE_MPATH by default would fix it. It will allow to reduce networking
feature gap to other operating systems. Linux and OpenBSD enabled similar support
at least 5 years ago.

ROUTE_MPATH does not consume memory unless actually used. It enables around ~1k LOC.

It does not bring any behaviour changes for userland.
Additionally, feature is (temporarily) turned off by the net.route.multipath sysctl
defaulting to 0.

Differential Revision: https://reviews.freebsd.org/D27428