Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC: 238366, 240924

Initial commit of an I/O provider for DTrace on FreeBSD.

These probes are most useful when looking into the structures
they provide, which are listed in io.d. For example:

dtrace -n 'io:kernel::start { printf("%d\n", args[0]->bio_bcount); }'

Note that the I/O systems in FreeBSD and Solaris/Illumos are sufficiently
different that there is not a 1:1 mapping from scripts that work
with one to the other.

This commit includes the fix so that our probes use "kernel"
instead of the Solaris specific "genunix"

Add DTrace's io.d, which handles tranlsations for file, buffer and
device info structures as well as the fds[] array. This is a raw
version of the file, unmodified, to be used as a baseline.