Require uint32_t alignment for ipfw_insn

[arichardson]

Summary

There are many casts of this struct to uint32_t, so we also need to ensure
that it is sufficiently aligned to safely perform this cast on architectures
that don't allow unaligned accesses. This fixes lots of -Wcast-align warnings.

Test Plan

fixes lots of compiler warnings

[arichardson]

sys/netinet/ip_fw.h

342

This could be _Alignas(uint32_t) with C11, but if we fall back to attribute((aligned)), we need an integer and can't use a type name.

[arichardson]

ping?

[ae]

I have some doubts, that this change is safe. But probably we will see the result quickly.