iproute2: tc: enable and fix support for using .so plugins

iproute2: tc: enable and fix support for using .so plugins

This enables using the tc module m_xt.so, which uses the act_ipt kernel
module to allow tc actions based on iptables targets. e.g.

   tc filter add dev eth0 parent 1: prio 10 protocol ip \
   u32 match u32 0 0 action xt -j DSCP --set-dscp-class BE

Make the SHARED_LIBS parameter configurable and based on tc package
selection.

Fix a problem using the tc m_xt.so plugin as also described in
https://bugs.debian.org/868059:

  Sync include/xtables.h from iptables to make sure the right offset is
  used when accessing structure members defined in libxtables. One could
  get “Extension does not know id …” otherwise. (See also: #868059)

Patch to sync the included xtables.h with system iptables 1.6.x. This
continues to work with iptables 1.8.2.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>