libxtables: Introduce global params structuring

libxtables: Introduce global params structuring

introduce a new struct,xtables_globals, so as to
localize the globals used and help in symbol renames.
The applications must invoke xtables_set_params() before starting
to use any iptables APIs.
xtables_set_params() is intended to free xtables from depending
(as it does right now) on existence of such externally definitions
(from iptables/iptables6 etc). At the moment, xtables wont even
compile without presence of at least one of {iptables/iptables6 etc}

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: Patrick McHardy <kaber@trash.net>