iptables: use different approach for managing loadable extensions

iptables: use different approach for managing loadable extensions

Since musl libc does not support unloading libraries via dlclose() and since
we should not explicitely call library constructors we need to use an
alternative approach to track the match registrations performed by iptables
shared objects.

This commit changes the iptables glue code to keep a global registry of non-
builtin matches and targets.

We implement the bookkeeping by intercepting xtables_register_match() and
xtables_register_target() calls in order to record any extension registration
attempt performed by a loadable iptables library.

The code subsequently uses the global list of dynamically loaded extensions
to re-register dynamic matches and targets for each address family / table
combination.

As a consequence we can get rid of the lib vector in the iptables handle
and remove the dlclose() handling entirely. This simplifies the
load_extension() as well.

Fixes FS#31.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>