Optionally disable building the kernel module.

Optionally disable building the kernel module.

Distributors (like Fedora) might be interested in including the ipset
tools and libs, but they often don't want to build and ship external
kernel modules, especially if those modules are already included in
their kernel packages.

This patch introduces a new --with-kmod configure option that can be
used to conditionally build the kernel module. The module is still built
by default, to preserve compatibility.

A user who wants to build only the user-space part of ipset can do so by
running the following:

    $ ./autogen.sh
    $ configure --with-kmod=no
    $ make
    # make install

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>