Convert ip6_forward() to the new routing KPI.

Update ip6_forward() internals to use deembedded IPv6 addresses
to simplify calls to the new KPI and prepare for the future
scope-embedding cleanup.

Add in6_get_unicast_scopeid() and in6_set_unicast_scopeid() scopeid
operation functions tailored for unicast processing.

Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D24334