libxtables: Move struct xtables_afinfo into xtables.h

libxtables: Move struct xtables_afinfo into xtables.h

The library "owns" this structure and maintains 'afinfo' pointer to
instances of it. With libxt_set, there's even an extension making use of
the data.

To avoid impact on library users, guard it by XTABLES_INTERNAL.

To eliminate the xshared.h include by libxt_set, DEBUGP has to be
redefined. Other extensions have similar defines, fix this later.

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