tun: ensure gc and argv are properly handled

tun: ensure gc and argv are properly handled

Everytime a argv object is initialized with argv_new(), it has
to be released with argv_reset() once not needed anymore.

Ensure this kind of objects are always properly released to avoid
memory leaks.

At the same time, remove those gc_arena objects that are initialized
but never used/released.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180613122824.4207-2-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17044.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>