Improve support for Equal-Cost Multi-Path (ECMP) routing

[AdSchellevis]

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Is your feature request related to a problem? Please describe.

Currently one can enable ECMP using the net.route.multipath, but our user interface isn't very supportive when using it
The idea of this ticket is to ensure ECMP can be used from static routes (without weight as FreeBSD doesn't support that) and settings are clearly communicated to the user.

Describe the solution you like

Support for ECMP in System: Routes: Configuration and, when possible, some way to cope with default gateways in the gateway settings.

Describe alternatives you considered

None

Additional context

sysctl:

net.route.multipath

Dump nexthop groups data:

netstat -4OnW
netstat -6OnW

https://reviews.freebsd.org/D24141
https://reviews.freebsd.org/D26449