routing: Implement merge of nhgrp in new multipath route

[pouria]

Summary

Routing subsystem allows creating new multipath routes by
nexthop groups (e.g RTA_MULTIPATH in netlink), in case of
a second nexthop group on the same route, don't panic and
merge the existing nhgrp with new one.

[pouria]

For tests, see D56190.

sys/net/route/nhgrp_ctl.c

836–838

For reviewers, it's meaningless to reference nhops of an nhgrp here like above. Therefore, I didn't.

843–845

For reviewers, don't worry about the reverse order here, it will eventually sorted by qsort.

[pouria]

For reviewers: this patch also fixes a panic caused by an incorrect assumption in nhgrp_get_addition_group().
That function assumes rnd->rnd_nhop is not an nhgrp_object, therefore sending two or more RTA_MULTIPATH attributes can trigger a panic.

[pouria]

This comment was removed by pouria.

[glebius]

sys/net/route/nhgrp_ctl.c

662

The parenthesis around sizeof() are superfluous.

[pouria]

Address @glebius comment.