<sys/kinfo.h>: Fix legacy inclusion issues.
Sadly this header was not being included properly for a long time.
Make it publicly accessible and put a big NOTE how to do it properly for
future codes. This makes the <sys/user.h> the only other header that
defines _KERNEL_STRUCTURES to solve long term inclusion order issues.
Previous variant was hiding implicit dependencies, adjust netstat(1).
Any changes in this header breaks a lot of ports, try not to change any
of the structs. Also make sure KERN_SIGTRAMP has public visibility.
While there remove two defines that were not used since introduced in
5dfd06ac148512faf075c4e399e8485fd955578f
UnifiedSplitRaw
