MFC: 238366, 240924

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"

git-svn-id: svn://svn.freebsd.org/base/stable/9@243989 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f