Remove a couple of unused macros.

MFC after:	3 days
Add a "translated type" argument to SDT_PROBE_ARGTYPE() and add some macros
which allow one to define SDT probes that specify translated types. The idea
is to make it easy to write SDT probe definitions that can work across
multiple operating systems. In particular, this makes it possible to port
illumos SDT probes to FreeBSD without changing their argument types, so long
as the appropriate translators are defined. Then DTrace scripts written for
Solaris/illumos will work on FreeBSD without any changes.

MFC after:	1 week
Update the SDT(9) man page with the macros added in 254468. Also change the
existing examples to not pass an mbuf as a probe argument. There's no
obvious reason to have it there, and it doesn't really jibe with the example
added in this revision.

MFC after:	1 week
Use sysctl(ICMPV6CTL_ND6_DRLIST) instead of SIOCGDRLST_IN6 ioctl.