JSON: Sort out rule position and handles in general

JSON: Sort out rule position and handles in general

First of all, don't print the position property when listing rules. This
was there only because libnftnl JSON output has it too, but since the
preferred way to *add* a rule at some location is via 'handle' keyword,
keeping "position" in output would be non-intuitive. Changing "position"
property name to "handle" instead is also a no-go since that would clash
with the real rule handle.

Secondly, turn all handle output on regardless of octx->handle setting.
For a programmatic API like JSON, this should be fine.

Thirdly, fix rule locations when parsing JSON: Respect "handle" property
for CMD_INSERT and CMD_ADD and ignore "pos" at all (actually even a
typo, should have read "position"). Also support "index" property
recently added to standard syntax.

Finally, adjust nft-test.py for the above changes: There is no
"position" property to drop from rule output, and "handle" property will
always be present.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>