add support for %lu in argv_printf and prevent ASSERT

add support for %lu in argv_printf and prevent ASSERT

%lu is not supported by our tiny argv_printf implementation, therefore
it will trigger an ASSERT() when parsing it at route.c:1638.

Add support for '%lu' in argv_print() and prevent the ASSERT from being
triggered.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180623021147.22792-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17115.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e38d3a004195f33c5c04fe7c04db5d66c53241bc)