libxtables: Introduce xtables_fini()

libxtables: Introduce xtables_fini()

Record handles of loaded shared objects in a linked list and dlclose()
them from the newly introduced function. While functionally not
necessary, this clears up valgrind's memcheck output when also
displaying reachable memory.

Since this is an extra function that doesn't change the existing API,
increment both current and age.

Signed-off-by: Phil Sutter <phil@nwl.cc>