Add a new cleaned up m_xt.c based on m_xt_old.c
The new m_xt.c has been updated to use the new names and new api
that xtables exposes in iptables 1.4.5.
All the old internal api cruft has also been dropped.

Additionally, a configure script test is added to check for
the new xtables api and set the TC_CONFIG_XT flag in Config.
(tc/Makefile already handles this flag in previous commit.)
Signed-off-by: Andreas Henriksson <andreas@fatal.se>