netlink: move NETLINK define to opt_global.h

Move the NETLINK define into opt_global.h so we can rely on it being
set correctly, without having to remember to include opt_netlink.h.
This ensures that the NETLINK define is correctly set. If not we
may end up with unloadable modules, due to missing symbols (such as
nlmsg_get_group_writer).

PR: 274306
Reviewed by: imp, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42179

tests: make ktest build on ppc.

MFC after: 2 weeks

Testing: add framework for the kernel unit tests.

This changes intends to reduce the bar to the kernel unit-testing by
introducing a new kernel-testing framework ("ktest") based on Netlink,
loadable test modules and python test suite integration.

This framework provides the following features:
* Integration to the FreeBSD test suite
* Automatic test discovery
* Automatic test module loading
* Minimal boiler-plate code in both kernel and userland
* Passing any metadata to the test
* Convenient environment pre-setup using python testing framework
* Streaming messages from the kernel to the userland
* Running tests in the dedicated taskqueues
* Skipping or parametrizing tests

Differential Revision: https://reviews.freebsd.org/D39385
MFC after: 2 weeks