Fix directly connected routes for "topology subnet" on Solaris.

This basically brings back the previously-working metric setting for
directly connected routes (add a "0" to the "route add" command) on
Solaris/OpenSolaris.  It was working in f0eac1a59790, and got broken
by the route.c/route.h refactoring in 576dc96ca1ef1 - the patch
un-does that change to the TARGET_SOLARIS code, plus adds some more
descriptive comments.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Josh Cepek <josh.cepek@usa.net>
Message-Id: <1364314175-20758-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7460
(cherry picked from commit 792e8956b999b6932d472e4ab592bff160e52888)