Major NPF improvements (merge from upstream):

- Switch to the C11-style atomic primitives using atomic_loadstore(9).

- npfkern: introduce the 'state.key.interface' and 'state.key.direction'
settings. Users can now choose whether the connection state should be
strictly per-interface or global at the configuration level. Keep NAT
logic to be always per-interface, though.

- npfkern: rewrite the G/C worker logic and make it self-tuning.

- npfkern and libnpf: multiple bug fixes; add param exporting; introduce
more parameters. Remove npf_nvlist_{copyin,copyout}() functions and
refactor npfctl_load_nvlist() with others; add npfctl_run_op() to have
a single entry point for operations. Introduce npf_flow_t and clean up
some code.

- npfctl: lots of fixes for the 'npfctl show' logic; make 'npfctl list'
more informative; misc usability improvements and more user-friendly
error messages.

- Amend and improve the manual pages.

branches: 1.1.10;
NPF improvements:
- Add support for dynamic NETMAP algorithm (stateful net-to-net).
- Add most of the support for the dynamic NAT rules; a little bit more
userland work is needed to finish this up and enable.
- Replace 'stateful-ends' with more permissive 'stateful-all'.
- Add various tunable parameters and document them, see npf-params(7).
- Reduce the memory usage of the connection state table (conndb).
- Portmap rewrite: use memory more efficiently, handle addresses dynamically.
- Bug fix: add splsoftnet()/splx() around the thmap writers and comment.
- npftest: clean up and simplify; fix some memleaks to make ASAN happy.