ktrace: Use designated initializers for the data_lengths array.

In the .o file, this only changes some line numbers (head amd64) because
element 0 is no longer explicitly initialized.

This should make bugs like FreeBSD-SA-14:12.ktrace less likely.

Discussed with:	des
MFC after:	1 week
remove two debugging messages, align comments with the code
in our development trunk
rate limit some error messages
align comments with the ones in our development trunk
whitespace change: fix one comment, remove a stale one.
misc bugfixes:
- stdio.h is needed for fprint()
- make memsize uint32_t to avoid errors due to overflow
- honor the *XPOLL flagg in NIOCREGIF requests
- mmap fails wit MAP_FAILED, not NULL.

MFC after:	3 days