zebra: refactor zebra_static_ipv4() and static_ipv6_func()

zebra: refactor zebra_static_ipv4() and static_ipv6_func()

This is a preliminary step to join both functions into one later.

The main idea here is to make these functions have separate arguments
for the nexthop address and the nexthop interface, and adjust the call
sites appropriately. Having an argument that could be a nexthop address
OR a nexthop interface was making the code very hard to follow. With
this simplification, a lot of code duplication was removed and now both
functions look very similar.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>