autotoolification of iptables/

This patch autotoolifies iptables/, and also does a couple of minor fixups.

I'm unsure about these warnings, though:

    src/libxt_ifgroup.c: In function 'ifgroup_parse':
    src/libxt_ifgroup.c:53:3: warning: implicit declaration of function 'xtables_check_inverse' [-Wimplicit-function-declaration]
    src/libxt_ifgroup.c: At top level:
    src/libxt_ifgroup.c:198:6: warning: no previous prototype for '_INIT' [-Wmissing-prototypes]

As far as I remember, `xtables_check_inverse()` is gone from recent iptables, but I may be wrong. As for the _INIT: do we need some -D magic in CPPFLAGS for that to go away? Or make the `_init()` functions `static void` with a `constructor` attribute?