Cast pointer to uintptr_t to avoid alignment warnings.

Both struct ip and struct udphdr both have an aligment of 2, but the
cast from struct ip to a uint32_t pointer confused GCC 9 into raising
the required alignment to 4 and then raising a
-Waddress-of-packed-member error when casting to struct udphdr.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D31941

tests/libalias: Factor out common primitives

Rework the tests to check the correct layer in a single test.
Factor out tests for reuse in other modules.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30412

tests/libalias: Add missing copyright statements

tests/libalias: Add perfomance test utility

In order to compare upcoming changes for their effectivness, measure
performance by counting opertions and the runtime of each operation
over the time. Accumulate all tests in a single instance, so make it
complicated over the time. If you wait long enough, you will notice
the expiry of old flows.

Reviewed by: kp (earlier version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30379

test/libalias: Tests for outgoing NAT

Testing LibAliasOut functionality. This concentrates the typical use
case of initiating data transfers from the inside. Provide a
exhaustive test for the data structure in order to check for
performance improvements.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30335