libxtables: Simplify pending extension registration

libxtables: Simplify pending extension registration

Assuming that pending extensions are sorted by first name and family,
then descending revision, the decision where to insert a newly
registered extension may be simplified by memorizing the previous
registration (which obviously is of same name and family and higher
revision).

As a side-effect, fix for unsupported old extension revisions lingering
in pending extension list forever and being retried with every use of
the given extension. Any revision being rejected by the kernel may
safely be dropped iff a previous (read: higher) revision was accepted
already.

Yet another side-effect of this change is the removal of an unwanted
recursion by xtables_fully_register_pending_*() into itself via
xtables_find_*().

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