Use more C99 initialization in add_route/add_route_ipv6().

Use more C99 initialization in add_route/add_route_ipv6().

This gets rid of a few #ifdef and also removes the need for
commit a11bea18b1c93 (argv is only initialized after the
early exit check on RT_DEFINED).

v2:
  use gc_new(), group gc and argv init in both add_route*() functions

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20210802152619.30754-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22694.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>